Encyclopedias
Encyclopedia Astronautica - http://www.astronautix.com/
The most complete resource for space history - rockets, spacecraft, launch vehicles, astronauts, cosmonauts, spaceflights, engines and space exploration.
Encyclopedia.Langenberg.com - http://encyclopedia.langenberg.com/
Provides easy to use direct links to many free encyclopedia and biography sites.
InfoPlease.com, Encyclopedia - http://www.infoplease.com/encyclopedia/
A current and comprehensive encyclopedia covering all the major branches of knowledge from astrophysics to Zimbabwe.
World Book - http://worldbookonline.com
Offers an encyclopedia, dictionary, atlas, homework help, study aids, and curriculum guides.