Reference
General  Lists  Converters and Calculators  Scale and Numbers

General:

Columbia Encyclopedia
Searchable, online edition of the Columbia Encyclopedia published by Columbia University Press and hosted by Bartleby.com.

Oddens' Bookmarks
Extensive list of maps, including earthquake, volcano, plate tectonics, ocean, and weather maps. Maintained by the University of Utrecht.

The Open Directory Project
Interactive Web directory project coordinated by Netscape. Includes extensive science and technology sections.

Wikipedia
Interactive encyclopedia project coordinated by the nonprofit Wikimedia Foundation. Includes extensive science and technology sections.