Usually I just use the synonym suggestions at dictionary.com (or its tie-in site thesaurus.com) but it's pretty lame. Most of the synonyms it suggests are basic and well-known words I already know. In the interest of writing novels, for instance, sometimes it's nice to be able to sprinkle in those more rarely used words to make the writing a little more artful, so to speak.
To that end, what's a good online thesaurus that might be able to help me expand my vocabulary or find words I might not normally think of?