The first few databases on this list are broad in scope, and are frequently used to start research. As your project advances, and as you need additional resources, use a variety of databases to expand your research.
Texas A&M University Library has digitized this series of maps and information published by the USGS between 1894 and 1945. Each folio in the series offers a topographic map and a geologic map of the 15' quadrangle, plus additional information particular to the map area. These maps sets offer great detail, in an easy to use online interface, although they are older.
Search for a geologic map data with keyword and map-based searching. Links are provided to online map images, where available, as well as text citations. Robust search tools allow queries of any place name or a lat/long search. Maps cover North America, primarily the United States.
Geologic map information from around the globe. Most maps are at approximately a 1:1,000,000 scale. Data are contributed by participating national geologic survey organizations. Coverage is still inconsistent, but growing.