Like Spotify — and to a lesser extent, Slacker and Pandora — Grooveshark is an online streaming-music service that supports both free users (with adverts) and premium/VIP ad-free accounts. But unlike those services, Grooveshark has been unsuccessful in getting an iPhone app accepted into iTunes. Apple has consistently rejected the app — possibly because Grooveshark's ability to sync playlists and favorites is a bit TOO iTune-ish.
So Grooveshark has now given up on iTunes — and have released their iPhone app to Cydia. This may be the first time in recent memory a legitimate, above-ground, commercially-motivated company has said "Hey, we want you to have our app — and if you have to jailbreak your iPhone to get it, we're cool with that."
(Still need a reason to jailbreak your phone? We've got a bunch of them…)