Many hardware synthesizers have done this (it's really common on keyboards, and I've also seen rackmount synth boxes and even MIDI drum modules that implement GM). So any synthesizer you might have that implements GM can play back the MIDI data from those files.
I forget the exact numberings but that's the idea. There's a standard ('General MIDI', GM for short) that gives numbers to common instruments, so that the synthesizer can know that some notes for instrument X should be played with a grand piano sound, but instrument Y refers to a violin sound.
MIDI is a set of instructional codes to tell the synth what notes to play and when.
