These days, the first thing I do after I login to my Mac is launch Twitterrific.
I like twitterrific because twitterrific integrated with growl notification system. Twitterrific is a simple desktop application, with a nice user interface, and is configurable. It has an iPhone version as well.
For Microsoft lover, a twitterrific clone call MadTwitter is available on Windows platform.
More twitter client can be found here.