I'm really happy that I've found an excuse to mention Bebot here in this space -- it might be my favorite app in the entire App Store. We've covered it before here on TUAW -- it's a musical instrument, basically, with the added bonus of having a super cute singing robot as a mascot. And I've never seen an app that makes such beautiful and simple use of the multitouch screen, allowing up to four voices to be played at a time by touching four different fingers to the screen in different places.
The reason I get to mention it here today is because the creator just added iPad support to the mix -- while it's pretty barebones, unfortunately (the graphic isn't quite as high resolution as you'd expect), the bigger screen will give you more room to spread out and find the synth notes you're looking for. And as always, even if you're not musical, it's just plain fun to play around on the tones by swiping your finger across the screen.
Bebot is now a universal app for just US$1.99. If you haven't had the pleasure of playing with this robot on your iDevice yet, definitely check it out.