iKaossilator is pretty nice. If you want to use your phone as a midi controller, to play with it using virtual instruments in a DAW, I have three app suggestions. After touch, which simulates X Y and Z axes, They can all be set to send a different CC message, so you could make the X axis the mod wheel, the Y axis the expression pedal, and the Z axis, if you don't change it, will send velocity information and aftertouch. You can customize the grid, by default, its set out like harmonically, but you can change it. I made a two octave keyboard with it by making two rows, and 12 columns, so its a little more than 2 octaves, and the two don't line up but I like it. You can also make smaller grids, and when you do, you have more room to tap in, so if you have a drum kit, this could work for you. This app has a few weirdnesses with Voiceover, but it is accessible, you will have to get used to them. One of these things is that when you are in a settings window, Voiceover will read the midi numbers and note names of notes under your finger, in other words, the settings dialogs do not open modally. This is good for most people, so you can make a change, then test it without needing to close the window, its not all entirely bad for voice over users, its just something you will need to be aware of. When you go to expression, this is where you can change what each axis does, there are X, Y, and Z vertically laid out on the screen to the left of the actual controls, above that is general. If you flick, you'll be flicking for the next century, so use explore. Another thing is that it has some buttons that are incorrectly labeled. You will see like I think 3 done buttons which appear at the toolbar at the top. The leftmost, at the top left of the screen opens your settings window. You will then find a button that may say Tamber Toy, Dueling Saws, or Network Session 1. This opens up for you to choose if you want the thing to use its own built in synths, or if you want it sending midi messages over your wireless network for your DAW to pick up. Which brings me to a bit of a tangent, you will need a program called RTPMidi if you're on a windows PC to act as an intermediary. It catches the MIDI messages from the network, and relays them to a virtual midi cable that you enable in your DAW. The next two done buttons are in between one button that can say one degree, octave, or page. The button in the middle controls how much the buttons to either side will change the grid. It will either transpose by a very small amount, an entire octave, or the entire length of your grid, so for me if I hit that it would transpose by presumably 13 notes, 1 plus the number of columns in my grid.Once you set the degree of change, the buttons to the left and right of that button actually do the transposing.
Another app is called TouchOSC, and it can act as a control surface, sending OSC messages, or it can send midi messages. It has two keyboards, but you don't really want to use them. This app has its own tool for interfacing with the PC, but you can still use RTPMidi. This app is good for just messing around with your instruments.
The final one is called knob lab, and it lets you do much the same. The app is free to download, but if you want to get more than one knob on the screen, you have to pay to unlock each layout that does that, or you can pay to unlock all of them at once, which I think was $2.99 USD.
The bipeds think this place belongs to them, how cute.