Looking for a dictionary and thesaurus for your Windows PC? I highly recommend the free WordWeb. (There's also a fuller-featured Pro version for US$18.) Since all the data is stored locally and not on the Internet like some other dictionary/thesaurus programs out there, WordWeb is fast and always available (not only when you're online). You can highlight a word in any application and then just click WordWeb's system tray icon or HotKey (Ctl+Alt+W is the default) and that word is instantly looked up and displayed (initially showing type, definitions, and synonyms). The user interface is top-notch too with buttons on the right to filter or jump to a specific type — noun, verb, adjective, or adverb. Tabs at the bottom let you quickly see synonyms, antonyms, attributes, or similar words.

I only wish they had this available for Mac as well!