Artha is a free cross-platform English thesaurus that works completely
off-line and is based on WordNet.

The main focus of Artha is high usability, with much simplicity. It has
distinct features like:

* hotkey press - word look-up (select text and press a hotkey for look-up)
* regex-based search (find words using wild-cards like *, ?, etc.)
* passive desktop notifications (of word definitions for uninterrupted
  workflow)
* spelling suggestions (when the exact spelling is vague/not known)