One iOS control surface that’s getting too little love in my poll on the left is TB MIDI Stuff.
It sends and receives OSC and MIDI, including NRPN, so you can do 14 bit high resolution MIDI. It even does bi-directional Sysex. The editor is built in so there’s no need to edit your layout on a computer then sync – add, delete and modify controls and pages right on your device. There are some built in control surfaces like a standard keyboard (with arp!) and an MPC type drum pad and many other editable templates.
Get it here for only $4.99 – daaaaamn that’s cheap – and I guarantee you will find uses for it. It’s worth that coffee-and-muffin price just for the default keyboard and MPC and the Wicki-Hayden templates, to say nothing of the custom templates you can put together quickly and easily. Oh, did I mention there are built in scales and customizable scales you can play from ribbons and other objects? Well, there are. It’s a Swiss army knife of MIDI features for a toothpick price.
Finally, here’s a video of one of the 1.x versions of TB MIDI Stuff (2.0 was recently released)