TweetDeck and Twhirl are specific to twitter but look also at "Spotify" - similar to itunes - which opts for a specialized client rather than a browser interface.
This may seem counter-intuitive. Who wants to run 5 applications in order to get access to web apps? The problem is that HTML over httpd is so limiting and challenging to engineering teams, that it actually may make sense to spend time supplying clients that work on different platforms rather than wading through css hell just to make an app look and act the same for the variety of browsers available.
This translates well to the PC. With broadband access becoming ubiquitous, we may see the end of the stateless web app. Really, the only thing preventing stateful apps from being developed was the HPPD protocol - which insists that everything delivered over the internet be ASCII - and the limited bandwidth available on the internet. That is obviously changing rapidly and broadband cell networks such as 3G and the smartphones which connect to them are pushing us closer and closer to a time when the browser may be a dinosaur rotting in the La Brea tar pits right beside the 56k modem.