TweetDeck is a popular Twitter client. I use it. And Phil Windley has included it in his Top Ten List of Valuable Web Sites and Services ("makes twitter easier to manage/use").
TweetDeck is an Adobe AIR desktop Flex application.
Wait a minute. Doesn't AIR *hurt* the web? Hmm.
Oh, right. AIR is simply a cross-platform application runtime with useful capabilities for running from and working with the web. Imagine.