Grooveshark mobile, created by nilezon, is an application able to stream any song from Grooveshark. The application is still full of bugs, so it should be considered more of a proof-of-concept at the moment.
UPDATE 29/9:
New alpha version (0.2) released!
Some of the crashing is hopefully gone.
Known bugs:
* BASS (the audio player) sometimes gives you a message box saying that the playback failed. Wait a few seconds and try again.
* The spinning umbrella gets stuck on the result list (scroll a little to remove it).
* The search query doesn’t get copied from the search screen to the result/player screen.
Requirements:
* Windows Mobile 6.1 (5.0, 6.0 and 6.5 will probably work).
* .NET Compact Framework 3.5.
* QVGA, VGA or WVGA screen resolution.
http://www.1800pocketpc.com/blog/wp-content/uploads/2009/10/GsMobile02.zip

