Both are worth every penny: Morphwiz (www.morphwiz.com) and Bebot Robot Synth (www.normalware.com). Bebot Robot Synth is the simpler (and cheaper) of the two at $1.99. You can customize the pitches and timbre that get played or choose from many preset functions, and you can also choose whether or not you want to see a grid of notes (and you can choose how many intervals you want shown).
Pitch goes from low to high as you move from left to right on the iPad and amplitude goes up and down as you slide from bottom to top on the iPad. The tones are great and the animated robot is pretty funny to watch.
Morphwiz is amazing. I'd been playing with it for over an hour and I don't think that I had gone through all of the crazy preset functions that it has. The price of $9.99 is well worth it for this dynamic and well-designed app.
Like Bebot, some of the presets show you the tone structures. You can shift octaves and customize the presets to your liking. The app was designed by people who know music and know sound (see Jordan Rudess of Dream Theater).
Header photo by Robert S. Donovan