The following links open in a new browser window.

Google
https://adwords.google.com/select/KeywordToolExternal

WordTracker
http://www.wordtracker.com

Keyword Discovery (Trellian)
http://www.keyworddiscovery.com
http://www.keyworddiscovery.com/search.html

Dictionary
http://www.dictionary.com

Thesaurus
http://www.thesaurus.com

Acronym
http://www.acronymfinder.com

Yahoo
http://inventory.overture.com
(may be down, or sometimes takes a long time to load).