Euphony


Euphony's web-based makes it the most accessible; no download or installation needed. Just open it up and select from a big list of bundled MIDI files to play around with. Or drag and drop in your own MIDI file to play it.

The app only bundles in piano sounds so by default, it can't be used to play a full orchestral MIDI piece. But it can be forked on GitHub and tweaked however needed.