FluidSynth is a real-time software synthesizer based on the SoundFont 2 specifications and has reached widespread distribution. FluidSynth itself does not have a graphical user interface, but due to its powerful API several applications utilize it and it has even found its way onto embedded systems and is used in some mobile apps.
A detailed chart of supported MIDI messages can be found in the wiki .
Calf Studio Gear - GNU/Linux Audio Plug-Ins