LMMS - Make music with a free, cross-platform tool

Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.

Ardour - Record, edit, and mix on Linux, Mac OS X, and Windows.

Korg Legacy Collection - Korg Legacy Collection is a suite of VST instruments that acts as an audio plugin software for integrating synthesizers and effects units into your digital audio workstation.

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.

Dexed - Dexed is a multi-platform, multi-format plugin synth that is closely modeled on the Yamaha DX7.