Aiksaurus is an English-language thesaurus that is suitable for integration with word processors, email composers, and other authoring software.
Another suggestion from Andre L.:
Aiksaurus is an English-language thesaurus. When you lookup a word it gives you synonyms. To interact with it directly use either aiksaurus (command-line) or gaiksurus (GTK GUI). It is also available as a library so other programs, for example abiword, can access it. One of the many great new packages coming in Sarge.
Upstream: http://www.aiksaurus.com (usually down) and http://aiksaurus.sourceforge.net/
More information on this package can be found on the Debian web site.
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Now available in RSS and ATOM flavors too.