Yahoo Education!
I use it all the time. You can simultaneously search:
- The American HeritageŽ Dictionary, Fourth Edition
- The American HeritageŽ Spanish Dictionary, Second Edition
- Roget's II: The New Thesaurus
- Columbia Encyclopedia
- Bartlett's Familiar Quotations
- Gray's Anatomy of the Human Body
- The Oxford Shakespeare (woot!)
- World Factbook
- Conversion Calculator
Just thought of one more, for anyone interested in German/English translation:
http://dict.leo.org. A great site, not just for vocabulary, but also for colloquial phrases and whatnot.