I've been using "twitter" lately, but there's a bunch of them. A lot of people seem to like twitteriffic.

And there's a free twitter app in the mac app store too, and I think that's better then using a browser tab. It integrates with growl, so you can get little pop-ups if you want them. It's all configurable. It's nice.

It's easy enough to try them all and get rid of the ones you don't like.

-David