Sparrow is a new and beautifully designed minimalistic Gmail client for Mac OS X that riffs on the UI principles of the iPad and OS X Twitter clients. Not sure which came first, but they're very close.

Won't be long before we'll see unification of all comms streams and feeds into on single UI / App paradigm like this. Totally different to the conventional 1997 vintage Microsoft Outlook panelled derivative UI we've all come to know and hate.