Wikipedia - http://www.wikipedia.org/ Wikipedia is a free content encyclopedia written collaboratively by people from all around the world.
Encyclopędia Britannica - http://www.eb.com/ Britannica offers a wide range of products for the reference and education market, including single- and multi-volume print titles, web sit subscriptions, and software.
World Book Online - http://www.worldbookonline.com The World Book Online Reference Center includes the complete World Book Encyclopedia, the World Book Dictionary, atlas, web links, magazines and historical documents