Encyclopedia of Earth: A free, fully searchable collection of articles written by scholars, professionals, educators, and experts who collaborate and review each other's work.
Geology.com: General geology site with information on a variety of topics. Includes a geological dictionary.
GeoIndex: search engine for geotechnology, environmental science,
hydrogeology, geology, mining, and petroleum geology.
Google: Geology: links to a number of sites dealing with geology. Includes links to Google directories on related disciplines such as geochemistry and geophysics.