OneLook’s reverse dictionary lets you describe a concept and get back a list of words and phrases related to that concept. Your description can be a few words, a sentence, a question, or even just a single word. Just type it into the box above and hit the “Find words” button. (Keep it short to get the best results.) In most cases you’ll get back a list of related terms with the best matches shown first. —Reverse Dictionary
My inbox is rather full at the moment, but I did manage to fish out this gem that Jim sent last week. Looks like it uses the content of wikipedia to fuel its searches.