I use Seesmic Desktop. I used to use Twhirl, which is put out by Seesmic as well, but decided I wanted the greater functionality of the Desktop version. It is very similar to Tweetdeck, which I have used -- but I like Seesmic much better. It has a great interface for my Mac.
TweetDeck (not had it crash on me yet since I started using it) all the way for me, I use the site from time to time (the new twitter layout is better for tweeting then the only one by far, but still is taking time to get use to it - which means I still prefer tweetdeck).