ReBirth for iPad
ReBirth for iPadis a $15 multitouch synthesizer that emulates the Roland TB-303 Bass synth and the Roland TR-808 and 909 drum machines. You can create music on it and output the songs as MP3s.
Combine these with FX units, fully featured pattern sequencers and a gorgeous-looking interface and you're ready to make killer tracks on your iPad. Share your music with friends on Facebook, Twitter and more using the built in sharing features.