The following sections contain all of the available sources of MIDI files that we've found thus far. For convenience, here's a table of contents:
Feel free to add sections as sources are found.
Our friend, midi_man, over at Reddit has provided us with a 130,000 Midi Collection. For the direct link to the download, check out the mega archive. The total size of the archive is 1.02 GB compressed and 3.65 GB uncompressed.
Once again, midi_man, comes in clutch with a massive collection of percussion samples. For the direct link to the download, check out the mega archive. The total size of the archive is 890.9 MB compressed and 3.36 GB uncompressed.
MIDI-sources is a GitHub repo which catalogs a lot of the information we're trying to capture here.