Encyclopedias
HowStuffWorks, Inc. - http://www.howstuffworks.com/
Explains hundreds of subjects, from car engines to lock-picking to ESP, using clear language and tons of illustrations.
Encyclopedia of Life Sciences (ELS) - http://www.els.net/
Features more than 4,300 specially commissioned and peer-reviewed articles. The articles within ELS are written by leaders in the field to provide comprehensive and authoritative coverage of this subject area and are aimed at researchers, students and teachers alike.