The original TweetDeck was an Adobe AIR application. Twitter bought it, and then rewrote it as a website. So it could be argued that they built it. There were

The original TweetDeck was an Adobe AIR application. Twitter bought it, and then rewrote it as a website. So it could be argued that they built it. There were alternatives before such as Seesmic, no idea how they work now, if they still do.