FluidSynth is used by many different applications on a wide range of platforms. A list of some of the applications can be found on the Wiki page Applications using FluidSynth.
FluidSynth has been used to create some original pieces of music and other audio. On the GitHub Wiki page List of creations you can find some examples.
FluidSynth does not have a graphical user interface, however many other programs that utilize it do, some of which are shown here.