FluidSynth is a real-time software synthesizer based on the SoundFont 2 specifications and has become widely used. FluidSynth itself does not have a graphical user interface, but thanks to its powerful API, several applications use it and it has even found its way into embedded systems and some mobile applications.

Qsynth - GUI for FluidSynth.

MuseScore is a WYSIWYG editor with the ability to quickly enter notes from both the computer keyboard and an external MIDI keyboard. Supports import and export of data in MIDI, MusicXML, LilyPond formats, as well as import of files in MusE, Capella and Band-in-a-Box formats. In addition, the program can export scores to PDF, SVG and PNG files, or to LilyPond documents for further fine-tuning of the score.