My favourite are
Mugician. Great for noise. Free & Wicked!
Soundrop. It's free, but I bought the upgrade just for fun.
RISCy. Also check Nick Collins other apps.
and finally a funny lil' synth: Sylo (as used by Gorillaz on their album The Fall)
There are many more and more pro and expensive. I will write some time in the future about them if you need more sonic toys in your iThing.
Mac Hip Hop - D-volution drum (also free)