Grooveshark is an online music search engine and music streaming service which allows users to search for, stream, and upload music free of charge that can be played immediately or added to a playlist with the option to cache files for offline play.
In the past, an iPhone app was available on the iTunes App Store, but was quickly removed as it appeared to violate Apple's iTunes Store Terms of Service. It has now returned on Cydia for free, however the app requires a VIP subscription ($3/month through Grooveshark). However, the app does allow for 50 free plays before the subscription service is required.
If you're using it, let us know how it's working for you!