Wikipedia
In all academic institutions, Wikipedia, along with most encyclopedias, is unacceptable as a major source for a research paper or speech. Other encyclopedias, such as Britannica, have notable authors working for them and may be cited as a secondary source in most cases. Wikipedia can provide you with general information and lead you to more credible sources.

Stanford Encyclopedia of Philosophy
This is a "dynamic encyclopedia" where each entry is maintained and kept up to date by experts in the field.

Index Encyberpedia
Funky site that provides links to a range of online encyclopedias, from the staid Britannica to some off-the-wall topics

Infoplease Encyclopedia
Access more than 57,000 articles from the Columbia Encyclopedia, Sixth Edition.