This excellent freeware program will stop you from ever fretting over a definition or antonym again. WordWeb offers hot-key lookup and offline resources in a smart package.
In addition to displaying definitions, synonyms, and antonyms, WordWeb can find sets of related words. The database has more than 150,000 root words and 120,000 synonym sets, and plenty of proper nouns, pronunciations, and usage tags. It can't tell you when you've split your infinitives or garbled your gerunds, but it gives great word accuracy on the fly.