If you use the finder in column view mode, there is a way to play an MP3 that is quicker than any app so far: simply clicking once on the mp3 file will display a play bar in the next column containing the entire track. It plays almost immediately. Some draw-backs: only one MP3 can be played at a time with this method, and only the front finder window will play sound. Still though, if you just want a quicky, this is pretty sweet.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010407133510783