Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Bosca Ceoil - A free, easy to use tool for creating music.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Renoise - Renoise is a digital audio workstation.
Omnisphere - Piano, pad and synth VST for DAW's.
Zynewave Podium - Podium is a modern digital audio workstation (DAW) for Windows.