Writing courses

Dictionaries:

Dictionary.com
http://dictionary.reference.com

English Dictionary
http://www.english-dictionary.us

Reverse Dictionary
(describe a concept and get back a list of words and phrases related to it)
http://www.onelook.com/reverse-dictionary.shtml

Thesaurus.com
http://thesaurus.reference.com

Encyclopaedias:
Encarta
Encyclopedia Britannica
Encyclopedia.com

Other:

Newsgroups

Search engines