SunVox - SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).
MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .
FamiStudio - FamiStudio is very simple music editor for the Nintendo Entertainment System or Famicom. It is designed to be easier to use than FamiTracker, but its feature set is also much more limited.
OpenMPT - OpenMPT is a popular tracker software for Windows.
MOTU Digital Performer - Get inspired, then refine your mix — all in a singular workflow.
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.