Sid Tracker 64 for iPad is an upcoming all-in-one chiptune studio! This looks like it is going to be big! Beyond emulating the iconic Commodore 64 sounds they've crammed a lot of features into this one, including Inter-App Audio and Audiobus. It was submitted yesterday to Apple for approval, so we should see this on the App Store soon!
There a couple of excellent videos for this, like an 8-bit remix of David Hasselhoff's True Survivor from the Kung Fury soundtrack. If you haven't seen Kung Fury yet, you should do so this weekend. It's only 30 minutes long and free.
Embedded here is the official preview video for Sid Tracker 64.