Probably some have found this by now, but as it's one of the most useful Dashboard widgets I've found, I thought I'd share it just in case: Symphonic. Simply type in a song and it'll start iTunes (if not running) and play it from your library. Works with typing in an album or artist as well (but always chooses the same song). It even has a certain amount of "fuzzy logic" for partial names. Saves me the time of bringing iTunes up and searching.