Acronym Finder
http://www.acronymfinder.com/

Research It! Language Tools
http://www.itools.com/research-it/

Rogets Thesaurus
http://humanities.uchicago.edu/forms_unrest/ROGET.html

Thesaurus.com
http://www.thesaurus.com/ ?

The Virtual Reference Desk Thesaurus
http://www.virtualref.com/thesaurus.html

Visual Thesaurus
Words show up as a "spatial map of linguistic associations," with clickable words.

that encourage threading.
http://www.plumbdesign.com/thesaurus/ ?

WordNet
http://www.cogsci.princeton.edu/