Roget's Thesaurus
WordSmyth English Dictionary--Thesaurus
Merriam-Webster Online
Dictionary Development Group
Doctor Dictionary
Hypertext Webster Gateway
All Dictionaries (monolingual, bilingual, multilingual)
Multilingual Dictionaries
Languages A-E Dictionaries
Languages F-H Dictionaries
Languages I-R Dictionaries
Languages S-Z Dictionaries
Other languages dictionaries
Britannica Online
Encarta Online
Cultural Encyclopedia
Funk & Wagnalls Encyclopedia
Grollier Multimedia Encyclopedia
Electronic Encyclopedia
Other encyclopedias

Theories and Practices of Lexicography

Australex (Offical site of the Australian Association for Lexicography)
USlex (Official site of the Dictionary Society of North America)
Euralex (Offcial site of the European Association for Lexicography)
International Journal of Lexicography (Oxford University Press)
Dictionary Research Center at the University of Exeter (DRC)