NASA offers the GeoCover website where global coverage imagery from 1990 and 2000 can be downloaded. This website also contains a tutorial on Landsat imagery and how to download and view them using free software:

The Blue Marble: Next Generation is a series of images that show the color of the Earth’s surface for each month of 2004 at very high resolution (500 meters/pixel) at a global scale.

The National Map Seamless Server is the ultimate location to explore and retrieve data. U.S. Geological Survey (USGS) and the EROS Data Centre (EDC) are committed to providing access to geospatial data through The National Map. An approach is to provide free downloads of national base layers, as well as other geospatial data layers. These layers are divided into framework categories: Places; Structures; Transportation; Boundaries; Hydrography; Orthoimagery; Land Cover; Elevation.

ISS Earth Knowledge Acquired by Middle (EarthKAM) school students is a NASA education program which enables students, teachers and the public to learn about Earth from the unique perspective of space. At the core of the program is a spectacular collection of digital images of Earth. The image collection and accompanying learning guides and activities are extraordinary resources to support classes in Earth science, space science, geography, social studies, mathematics, communications and even art.  ISS EarthKAM images are unique because they are taken by middle school students. Using the World Wide Web, select middle schools request images based upon their classroom investigations. Since the program's inception in 1996, the ISS EarthKAM camera has flown on five Space Shuttle flights and taken almost 5000 images. Currently, the camera resides on the International Space Station and has taken a variety of pictures.

USGS-EROS; The Earth Resource and Observation Science Centre of the U.S. Geological Service offers a wealth of Aerial and Satellite Imagery. Most of this information is oriented on North America: 

TerraServer has assembled the largest variety of aerial photos, satellite images and USGS topo maps on the Internet. Users can search and view all of their online imagery and then purchase a digital image download or a high quality print.

ResMap provides free online access to GIS-ready satellite data. Imagery covering the entire globe is accessible, with resolutions ranging from 1 km to 14.25 metres for most areas, and in some selected regions 1 metre imagery is available. Over 10 Terabytes of LANDSAT imagery is available for direct and free use in GIS/CAD/RS/MS Office applications on your desktop without the need to download the data.

Land Processes Distributed Active Archive Center (LP DAAC)
The Land Processes Distributed Active Archive Center (LP DAAC) is a component of NASAs Earth Observing System (EOS) Data and Information System (EOSDIS). LP DAAC processes, archives, and distributes land data and products derived from the EOS sensors. Located just outside Sioux Falls, South Dakota, the LP DAAC handles data from three EOS instruments aboard two operational satellite platforms: ASTER and MODIS from Terra, and MODIS from Aqua. ASTER data are received, processed, distributed, and archived while MODIS land products are received, distributed, and archived. Both data sets are vital contributors to the inter-disciplinary study of the integrated Earth system.

The Global Land Cover Facility (GLCF) provides earth science data and products to help everyone to better understand global environmental systems. In particular, the GLCF develops and distributes remotely sensed satellite data and products that explain land cover from the local to global scales. Primary data and products available at the GLCF are free to anyone via FTP. Online datasets may be accessed electronically through the Earth Science Data Interface (ESDI).

The National Geospatial-Intelligence Agency (NGA) is a Department of Defence combat support agency and a member of the national Intelligence Community (IC). NGA develops imagery and map-based intelligence solutions for U.S. national defence, homeland security and safety of navigation. NGA provides timely, relevant and accurate geospatial intelligence in support of national security objectives. The term "geospatial intelligence" means the exploitation and analysis of imagery and geospatial information to describe, assess and visually depict physical features and geographically referenced activities on the Earth. Geospatial intelligence consists of imagery, imagery intelligence and geospatial (e.g., mapping, charting and geodesy) information. Information collected and processed by NGA is tailored for customer-specific solutions. By giving customers ready access to geospatial intelligence, NGA provides support to civilian and military leaders and contributes to the state of readiness of U.S. military forces. NGA also contributes to humanitarian efforts, such as tracking floods and disaster support, and to peacekeeping.

The Comprehensive Large Array-data Stewardship System (CLASS) is an electronic library of NOAA environmental data. This web site provides capabilities for finding and obtaining those data. CLASS is NOAA's premier on-line facility for the distribution of NOAA and US Department of Defence (DoD) Polar-orbiting Operational Environmental Satellite (POES) data, NOAA's Geostationary Operational Environmental Satellite (GOES) data, and derived data.

The VGT4AFRICA project aims at setting up an operational and timely distribution system of VEGETATION data from the SPOT satellites and high level derived products to all African countries. The goal is to set up, in the framework of GMES, an operational capacity based on the SPOT VEGETATION system in support to sustainable development policies in Africa.

Dundee Satellite Receiving Station, Dundee University, UK, maintains an up-to-date archive of images from NOAA, SeaStar, Terra and Aqua polar orbiting satellites. Images from geostationary satellites covering the whole earth are also available. To view the images you must register. Registration is free and the images are free.

The CGIAR-CSI GeoPortal provides SRTM 90m Digital Elevation Data for the entire world. The SRTM digital elevation data, produced by NASA originally, is a major breakthrough in digital mapping of the world, and provides a major advance in the accessibility of high quality elevation data for large portions of the tropics and other areas of the developing world. The SRTM digital elevation data provided on this site has been processed to fill data voids, and to facilitate it's ease of use by a wide group of potential users. This data is provided in an effort to promote the use of geospatial science and applications for sustainable development and resource conservation in the developing world. Digital elevation models (DEM) for the entire globe, covering all of the countries of the world, are available for download on this site. The SRTM 90m DEM's have a resolution of 90m at the equator, and are provided in mosaiced 5 deg x 5 deg tiles for easy download and use. All are produced from a seamless dataset to allow easy mosaicing. These are available in both ArcInfo ASCII and GeoTiff format to facilitate their ease of use in a variety of image processing and GIS applications. Data can be downloaded using a browser or accessed directly from the ftp site.

Landsat.org supports the purchasing, distribution, and sharing of Landsat TM (Landsat 4 and 5) and ETM+ (Landsat 7) imagery worldwide by providing a simplified, platform-independent user interface and search engine with online data ordering. Landsat.org supports research centers, science teams, and educational organizations by providing customized search interfaces, access to data hosting services, clearinghouse services, data brokering, and imagery cooperatives.

Remote Sensing data for Brazil and its region. The CBERS Program was born from a partnership between Brazil and China in the space technical scientific segment. Consequently, Brazil joined a select group of countries with remote sensing technology. Brazil monitors its huge territory by its own remote sensing satellites, looking forward to consolidate an important autonomy in this segment.
http://tinyurl.com/23ufgn

The U.S. Agency for International Development (USAID) Famine Early Warning Systems Network (FEWS NET) is an information system designed to identify problems in the food supply system that potentially lead to famine or other food-insecure conditions in sub-Saharan Africa, Afghanistan, Central America, and Haiti. FEWS NET is a multi-disciplinary project that collects, analyzes, and distributes regional, national, and sub-national information to decision makers about potential or current famine or other climate hazard-, or socio-economic-related situations, allowing them to authorize timely measures to prevent food-insecure conditions in these nations. Regions and countries with FEWS NET representatives include sub-Saharan Africa (Angola, Burkina Faso, Chad, Djibouti, Ethiopia, Kenya, Malawi, Mali, Mauritania, Mozambique, Niger, Nigeria, Rwanda, Somalia, (southern) Sudan, Tanzania, Uganda, Zambia, and Zimbabwe), Central America (Guatemala, Honduras, and Nicaragua), Afghanistan, and Haiti.

The Asia Pacific Natural Hazards Information Network (APNHIN), hosted by the Pacific Disaster Center (PDC), directly supports disaster and resource managers, planners, governments, and nongovernmental organizations with a suite of applications and information services designed to search for, evaluate, and access high-quality geospatial data for natural hazard applications within the Asia Pacific region. APNHIN comprises a community of organizations who create and share disaster and hazards-related information.

The website for the Advanced Land Observing Satellite (ALOS), which was launched by Japan Aerospace Exploration Agency in January 2006. ALOS has three remote-sensing instruments, PRISM is a panchromatic radiometer with spatial resolution in order to obtain terrain data including elevation. AVNIR-2 is a visible and near-infrared radiometer for observing land and coastal zones and provides better spatial resolution. PALSAR is a phased array type L-band Synthetic Aperture Radar, an active microwave sensor for cloud free and day and night land observation.

The GIS Data Depot houses data in support of the GIS industry. The majority of the data provided has been downloaded by their staff from a wide range of GIS Web sites located on the Internet. There is also value added data where they have performed some translation, attribution, analysis, or other data enhancing operations. By providing all of these resources from one convenient location, GeoCommunity has made every effort to ensure that you can locate the data users are searching for in a quick and efficient manner.

Terrainmap.com: Terrain Modeling and Mapping Techniques using Digital Elevation Model, SRTM, SDTS, GLCF, DRG, DLG, DTED ASTER, Landsat, EarthSat, EarthKam, World Wind and JPL Data

EOS-WEBSTER provides free, customized Earth Science data. Data holdings can be searched spatially, temporally, or by keyword, freetext, or collection name. They have distributed almost 700,000 data products to our users world-wide since 1999.

EuDASM is the European Union Digital Archive on Soil Maps of the World

The Geography Network is a global network of geographic information users and providers. It provides the infrastructure needed to support the sharing of geographic information among data providers, service providers, and users around the world. Through the Geography Network, you can access many types of geographic content including dynamic maps, downloadable data, and more advanced Web services.

ERDAS Image Web Server is a high-speed, specialized application that efficiently distributes massive amounts of geospatial imagery to thousands of users, all on a single server. Solving the infrastructure congestion problems traditionally associated with deploying large amounts of image data, users quickly access the information they need. With ERDAS Image Web Server, individuals may access imagery using CAD, GIS, mobile, web and desktop applications.

The objective of this Geosuccess (Global Earth Observation in Support of Climate Change and Environmental Security Studies) website is to demonstrate a variety of operational products based on remote sensing data, which give timely information on the evolution of the vegetation cover and its related parameters. Some of these products are calculated in near real time, at the global or regional scale.

EUMETSAT provides cost-effective operational satellite data, services and products in response to the needs of its users. This is achieved through the establishment and operation of satellite systems and effective dissemination schemes, and by maintaining and developing the existing services and enhancements to meet new requirements.