A simple to use Beat Making app. I find it very fun. You can export sounds you create to soundcloud. It only works in 4/4 and 8 bar loops, but nevertheless it's very entertaining.

An extremely cool and simple to use Synthesizer app. $1.99 on the App Store, and even for just messing around I found it worth the money. Very good app.

Skip to like page 12. Good info on developing relative pitch and learning intervals.

I don't care if this has nothing to do with interval ear training, I just found what this guy had to say about perfect pitch really interesting. I still have no idea if my pitch is actually perfect or not though haha.