I am not a native English speaker so I often use thesaurus when writing a blog post or just an email message. Long time ago I always used web based solutions but since I found Artha - The Open Thesaurus I wouldn't change it for anything else as it has handy and slick user interface, on the fly search, suggestions and hotkey lookup.
It uses free open lexical database for English (WordNet) so it doesn't need an internet connection and is completely Open Source.
To install Artha on a Debian system execute the following command:
$ sudo apt-get install artha
Check out hotkey lookup as it is my favorite feature.