Zerberus, The MuseScore SFZ synthesizer
Since MuseScore 2.0, MuseScore includes 2 synthesizers: One for SF2/SF3 named Fluid and one for SFZ named Zerberus.
Zerberus was first implemented by Werner Schweer in 2013/2014 for MuseScore 2.0 and it was only supposed to play the Salamander Piano SFZ back then. Zerberus has been greatly improved by hpfmn during Google Summer of Code 2016. He documented the supported opcodes, added support for more opcodes, tested it with several free SFZ and even experimented with Disk Streaming
The SFZ format is a plain text format first documented in "Cakewalk Synthesizers: From Presets to Power User" by Simon Cann (Chapter 13). The LinuxSampler project started an open documentation but it's far from complete. Also sfzformat.com has the beginning of the documentation.
An important note : several popular SFZ players added more non documented opcodes. In particular, the Aria Engine (see below) implements several custom opcodes that are popular among SFZ creators.
Free SFZ synthesizer
Popular non free SFZ player
The original SFZ by RCG audio and SFZ+
Aria Engine is a popular SFZ player engine. It's used in particular in Sforzando a free as in beer player available standalone and as VSTi/AU/RTAS/AAX . The Garritan Orchestra, now owned by MakeMusic, is also powered by the Aria Engine.
Kontakt by Native Instrument can import SFZ files but my (lasconic) understanding is that they are converted to Kontakt proprietary format in order to be played.