SoundGrail - SoundGrail Music App designed for pianists, guitarists, and DJs.
FL Studio - Image-Line's FL Studio, now on it's 12th version, is a well-known music production suite and the most popular beat processor on the market, due no doubt to its longevity. Read more about FL Studio.
Splice - Music creation, collaboration, and sharing made simple.
Caustic - Caustic is a music creation tool inspired by rack-mount synthesizers / samplers rigs.
keezy - A colorful soundboard. Play with music.
n-Track Studio - Audio recording and music creation app for iOS.