There are a number of free web-based thesauri, including:
In addition to online resources, the GNOME Dictionary program is available under the Applications menu in the Office catgeory, or from the command line as gnome-dictionary.
You can type the name of the word to look up in the box provided, and press Return. You can then press "Ctrl-T" to view similar words, using the free online Moby Thesaurus.
The older "thesaurus" program is available in the sipb locker. It's a command-line program, so in your Terminal window, you would type: