TouchDAW - Full-featured DAW control and general purpose MIDI control app.
Reaper - Reaper is a focused digital audio workstation (DAW) developed by Cockos. In the creation of the software, the digital audio technology company intended to make audio editing accessible to the masses.
XotoPad - MIDI instrument controller with chords, scales, faders, and more for multi-touch Windows devices.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Midi Studio - Full-featured, modular MIDI control surface for iOS
Ardour - Record, edit, and mix on Linux, Mac OS X, and Windows.