eledrumPOP (new iphone drum synth) currently free.
First of all, I haven't tested it, it requires ios 6.0. There is an iPad version that costs 4.99. I don't know if this will stay free or not; I don't know itf it's any good. The developer is Japanese, so at least it should be interesting
If you want to friend me on GC,tell me you're from TA and include your screen name.