Twitter tools, Twitter apps, Twitter clients; what ever you want to call them, are light-weight, unobtrusive applications that stay open on your desktop conveniently allowing you follow incoming Tweets as well as post them. These applications can simplify Replying and ReTweeting, as well as aid in organizing and tracking messages.
Many of these Twitter applications are free and available for a variety of platforms including Windows, Macintosh, as well as Windows Mobile, BlackBerry, Android, Palm and iPhone. I have listed some of the more popular ones below. Since many are free you can play around and see which works best for you.
Windows/Mac
Seesmic
Tweetdeck
Twirl
Twitterific
iPhone
Android
BlackBerry
Windows Mobile