PandoraJam is a desktop application for the online music service Pandora, with nice advanced features.
Unfortunately, since is now only available to the UK and US, lots of users will shy away from it and turn to Last.fm, which also provides a good desktop playback client. You'll also be able to record songs playing in PandoraJam to play them in your iPod or iTunes. The program adds wireless streaming to Airport Express, which although unstable every now and then, should work well if you have a good wireless connection.
PandoraJam is much more than a simple desktop application for Mac though. PandoraJam will show status updates in Skype, Adium and iChat but not other IM clients. You can set the autoscrobble feature to submit your songs to Last.fm and use Growl to get popup updates on new songs in your list.
The application is fitted with a nice interface which displays a list of your stations, the basic playback buttons and a handy search bar. Pandora is one of the best online music services you'll come across and, thanks to PandoraJam, you'll be able to enjoy it straight from your desktop.