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Qsynth, a front end for fluidsynth

FluidSynth is a real-time software synthesizer based on the SoundFont 2 specifications.

FluidSynth itself does not have a graphical user interface (GUI), however, there are frontends that exist for it, eg:

Since version 5.4, Ardour has a built in plugin called 'a-Fluid Synth' which uses FluidSynth as it's backend.