Here's a tidy little freeware program that is commercial quality. It has 120k+ words in the word list, which are all linked in various ways (i.e. word web).
This little proggie is perfect for writers, because (1) it's small and very stable; (2) the dictionary and thesaurus are tightly integrated; and (3) differences in scope of the definitions are tabbed (try it and see what I mean).
The dictionary definitions themselves are complete but brief, not as full as a full blown dictionary program like Collins or Websters or Oxford (I have tried all those too) and without etymology. This is adequate for writing because I don't need a complicated definition, and I'm more interested in the thesaurus than the dictionary portion. Most of the bigger and more expensive programs separate the dictionary and thesaurus. And most have a clunky interface.
The Pro version adds more flexible searches (wild cards) and customizable word lists. The Pro version is only $20 US.