Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.
Keezy Drummer - Make a beat in seconds
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.
Sampulator - Make (and record) beats on your keyboard
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.
Auxy Music Studio - Auxy brings real music creation to iPhone in a simple and inspiring format.