A real-time software synthesizer based on the SoundFont 2 specifications. It can be used for live audio synthesis of MIDI events or for rendering MIDI files to disk, using SoundFont instruments. It can also be embedded in other applications.
This is not a application with a graphical user interface, however there are several GUI applications which utilize FluidSynth.
iConnectMIDI PortManager for Windows, Linux and Mac
iCoCo is a Windows/Linux/Mac application for configuring the iConnectMIDI ("iCM") MIDI router by iConnectivity. It utilizes the awesome portmidi library and the brilliant FLTK GUI toolkit.