Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
MilkyTracker - MilkyTracker is an open source, multi-platform music application for creating .MOD and .
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
OpenMPT - OpenMPT is a popular tracker software for Windows.
Omnisphere - Piano, pad and synth VST for DAW's.
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.