Check out this really awesome Commodore 64 MSSIAH Demo:
The music and sound was done by Mr. Woofer on a real C64 computer using the MSSIAH MIDI cartridge and SID software.
The MSSIAH is a MIDI cartridge for the Commodore 64. It contains a suite of music applications that starts instantly as you insert the cartridge and start up the computer. With these applications you can play the C64’s audio circuit (SID) via MIDI or stand-alone with the internal sequencers.
Source: MSSIAH – MIDI SID Software
MSSIAH is short for MIDI SID Software Integrated Applications and Hardware and means that the cartridge contains both software and hardware to midify the C64. Since they are integrated you won’t need hard-to-get MIDI peripherals to hook it up to your synthesizer or sequencer.