Web 2.0 services that can potentially provide rich, abundant, and timely flows of geographic and geo-referenced information. Collectively they might be termed volunteered sources. They include geotagged entries in Wikipedia, the more specialized place descriptions accumulating in Wikimapia, sites such as OpenStreetMap that support volunteer efforts to create public-domain geospatial data layers, the geotagged photographs of Flickr and Panoramio, and mashups with Google Earth and Google Maps. (source: Michael F. Goodchild, UCSB)
 

OpenStreetMap is a project aimed squarely at creating and providing free geographic data such as street maps to anyone who wants them. The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive or unexpected ways

Google Earth is a free-of-charge, downloadable virtual globe program. It maps the earth by superimposing images obtained from satellite imagery, aerial photography and GIS over a 3D globe.

WikiMapia is an online map and satellite imaging resource that combines Google Maps with a wiki system, allowing users to add information (in the form of a note) to any location on earth.

P3DM Where? is an interactive world map based on Google Map technology operated collaboratively by participatory mapping practitioners and serves at locating "participatory 3D Modelling (P3DM) exercises" worldwide.

Panoramio is a geolocation-oriented photo sharing website. Currently, some of the photos uploaded to the site can be accessed as a layer in Google Earth, with new photos being added at the end of every month. The site's goal is to allow Google Earth users to learn more about a given area by viewing the photos that other users have taken at that place.