Wikipedia
The free-content encyclopedia that anyone can edit. There are also other national editions, e.g. German.

Encyclopædia Britannica
Well, who doesn't know about it ...

Encarta Worldwide
There you will find all national editions online, e.g. USA, UK, Australia, etc.

xrefer
Calls itself "the web's reference engine". xrefer contains encyclopedias, dictionaries, thesauri & books of quotations. All entries are cross-referenced.