Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Guitarix - Guitarix is a simple Linux Rock Guitar Amplifier for https://alternativeto.
Omnisphere - Piano, pad and synth VST for DAW's.
GNUitar - GNUitar is guitar effects software that allows you to use your PC as guitar processor.
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.
Ampire - State Space Modeled guitar amp/cab/stompbox simulator.