Namely SunVox and Klystrack. Both have playback libraries and both generate small files (usually less than 100kb unless you use samples) while sounding quite nicely. This means you can cram much more music into your game and still make it easy on people’s bandwidth (extremely important for RPGs which tend to have a lot of music). Or just have smaller output file in general, which is useful for e.g. gamejams with arbitrary size restriction.