One of the the Twitter clients I started looking into is Hahlo. It's basically a web based application used by a lot of iPhone users. You can make it into a great desktop Twitter client using the combo of Fluid, and Growl.

This screencast will give you the steps to create the Hahlo twitter client complete with growl notifications.

Links:

Fluid
Hahlo
Growl
Hahlo icon
Hahlo user script

Additional links on this topic

Lifehacker
MacTips
Fluid Blog