OmniMIDI (Formerly Keppy's Synthesizer) is a free and Open Source MIDI driver for professional use. It is a fork of the original BASSMIDI Driver by Kode54.
The author warns that it's for professional use meaning advanced users. The issue could arise where one single driver function, like this one, could break the entire audio stream, and you might not be able to repair it.
OmniMIDI's driver has unique features including:
Automatic rendering recovery. The driver will always try to give you the best audio quality, no matter what MIDI you're trying to play.
Sparse user interface: No fancy graphics can distract the user from the original purpose of the driver and designed for people who aim for features more than for style.
The ability to use up to 4 cores/threads, to ensure each function is executed at its best. Each core hosts a vital part of the driver: The first thread hosts the settings loader, the debug info writer, etc., the second hosts the MIDI event parser, the third hosts the audio render, and finally, the fourth hosts the ASIO driver (When using the ASIO engine).
Constant updates: to keep the driver fresh and always up-to-date to users requests.
OmniMIDI has become the go-to MIDI driver for anyone who used to use BASSMIDI Driver years back. OmniMIDI has everything that BASSMIDI Driver had and a lot more with a relatively easy to use interface.