While the web rules, there’s still room to play on the desktop.
One example is Twhirl – a social software client that lets Twitter users take their conversations out of the browser and onto the desktop. Built on the Adobe Air platform, Twhirl functions like your IM chat client but gives you all the capabilities and features of the Twitter platform – you can tweet, receive other tweets, look up user profiles, or cross post to other social platforms like Pownce and Jaiku.
Twhirl is a free software, compatible with both Mac OSX and Windows systems. You will need to download Adobe Air, a development tool and environment that lets you utilize web technologies, like XHTML/CSS and Flash, in desktop software. It too is a free download.