Kontakt - With KONTAKT 6, you can sound like a snare drum, a symphony orchestra, or anything in between, from a universe of sampled instruments.
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Pianoteq - Pianoteq is a virtual piano that runs on your computer.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
Magix Independence Free - Magix Independence Free is a sampler software that allows you to create any type of sound with its 2Gb library.
Omnisphere - Piano, pad and synth VST for DAW's.