Snitter is a tool built in Adobe AIR to send Tweets to your Twitter stream. It was created by a Canadian, Jonathon Snook, who wanted certain features in Twitter that didn't exist.

I've used Twitterific quite a bit, and there are issues with it I don't like. It is a little bit of a challenge to shut off--to quite out of the application you have to go into the application's configuration menu which strikes me as a little odd. With Snitter, you go to your regular menu and quit in a pretty standard way. Twitterific does a fine job of allowing you to post to Twitter--but it is only MacOS friendly. Snitter is available for Windows and MacOS.