National Geologic Map Database. Its Map Catalog lets you search for 90,000+ maps and reports, whether by keyword, theme, location, or publisher. Over 600 publishers are represented, including the geological surveys of the states (past and current) as well as many universities and societies and private publishers.
Many maps can be downloaded; for those that are unavailable online, you'll be given contact and ordering information.
Other resources include MapView, TopoView (in beta), and Stratigraphy, which is under development.