A good freely accessible encyclopedia is encyclopedia.com - provides users with more than 57,000 frequently updated articles from the Columbia Encyclopedia, Sixth Edition. Each article is enhanced with links to newspaper and magazine articles as well as pictures and maps - all provided by eLibrary. Included are dictionary and thesaurus.
Why not try out the famous Encyclopedia Britannica Online.
Microsoft have a respected encyclopedia called Encarta originally available on CD in multimedia format, it is now available on-line at MSN Encarta.
Alternatively try Wikipedia, a free content encyclopedia available in many languages that anyone can edit.
The offering from publisher Pearson (owner of the famous Penguin books) is Infoplease, their site includes encyclopedia, almanacs, atlas, dictionary and thesaurus.
Encyclopedia Mythica is an online encyclopedia of myth, folklore and legend
To find out about searching, and downloading free software and stuff from the internet, Freesoft offers a good guide.
For information on all major theories: The Internet Encyclopedia of Philosophy has a good listing, updated regularly and contributed to by actual philosophers on the internet.
If you like tinkering with your PC or car engine or simply interested in how things like jet engines work HowStuffWorks provides a comprehensive encyclopedia of science and technology in general.