RapMushi is an iPhone app made by Tsubasa Naruse that lets you create rap music by pushing various buttons, much like on a soundboard. You start by selecting a beat and you can choose between four different styles, two hip hop, one dub and one electro. On top of that you can add different extras like piano and guitar. When you’ve chosen your beat, all you have to do is start rapping along with the rhythm. It comes with 54 awesome vocal buttons and if you feel like it, you can set it to rap automatically while you do some scratching.
The app is free and a lot of fun to play with so I definitely recommend it. Also check out Samurai Boy which is pretty much the same app but with new beats and vocals and better sound.