It's a software synthesizer based on SCC and OPLL made by Mitsutaka Okazaki in 2004, later maintained by Chris Moeller. This is slightly updated version which can be built and run on Linux and macOS as well as it worked on Windows originally.
This is a preserved and slightly updated project, no longer maintained for a while.
There is a NEW project called libEDMIDI, which is a direct successor of this: