The Moby Thesaurus II "contains 30,260 root words, with 2,520,264 synonyms and related terms", and is public domain: https://en.wikipedia.org/wiki/Moby_Project#Thesaurus
The word lists can be downloaded on Project Gutenberg: http://www.gutenberg.org/catalog/world/results?title=moby+li...
There's also some public domain software associated with it: https://github.com/words/moby