This morning I'm releasing the source code for Kappa, which is a PyObjC based twitter client for OSX. It has become my default twitter client, not necessarily because it is an excellent twitter client, but more because the needs of a twitter client are quite limited. I attempted to do a couple of creative things with the user interface (allowing to filter tweets by regular expression, and a progress bar to show the next time tweets will be retrieved, different display mode when front and not front), but overall their value has been limited, if not negative.
Feel free to play around with this, and contribute back if you feel like it. Since the basics are already all implemented (except for making links clickable, just never got around to that), you should be able to play around with a personalized twitter client pretty easily.