I don't think I've ever fallen in love with an app as quickly as Bebot. Not only is it a pretty darn powerful synthesizer with an interesting touchscreen interface, but that robot. That robot! He makes you want to play something, anything, with those cute little closed robot eyes and that dashing tux and that knowing smile. If you just want to make funny robot noises, that's easy enough, but for the musically inclined, there's a really surprising amount of features under the hood, including analog filters, a delay/looping effect, chorus and overdrive effects, and even an autotune system. Oh, and because of the multitouch screen, you can play up to four waveforms at a time. In the hands of someone who knows what they're doing, this thing can be really impressive.
It's available from a company called Normalware (any app with that robot in it has basically sold me from now on -- there's even a t-shirt of the guy) on the App Store right now for $1.99.