m. c. de marco: To invent new life and new civilizations...

There are plenty of free on-line dictionaries and thesauri out there, but the selection is limited once you go off-line. WordNet is an interesting variation on the thesaurus theme. You can get a MacOS X package from Sourceforge.

JDict, is a dict-protocol dictionary server you can run locally for your own use. The easiest way to install JDict on MacOS X is with DICTatoro. The standard interface is a local web page, but the Mac version also includes a special configuration of OmniDictionary and a nice selection of free dictionaries.

Update

DICTatoro and OmniDictionary appear to no longer be maintained, but you may be able to find an old copy if you search hard enough, and have an old enough computer to run them on.