Twhirl is designed to enable you to get Tweets automatically fed through to your desktop, a bit like using a typical Instant Messaging service. You can also post Tweets using Twhirl as well. Most of the features available on the Twitter website are accessible through twhirl, too.

Here is the blurb from their own website:

• runs on both Windows (2000/XP/Vista) and Mac OSX
• connects to multiple Twitter and Friendfeed accounts
• notifications on new tweets
• shorten long URLs (using snurl, twurl or is.gd)
• cross-post updates to Pownce and Jaiku
• post images to TwitPic
• search tweets using TweetScan
• timeline filtering
• localized to English, German, Italian and Spanish
• color schemes
• automatically check for new versions

There are a lot more things twhirl can do to improve your twitter experience, and it offers various configuration options to adapt to your personal needs.