midiworld's Composers' Pages - Lots of original MIDIs by people you probably don't know, for example, check out the mind-blowing weirdness of zha, or visit the midifiles index for more cool jazz, classical, and early music files.
The Very Best MIDIs, it is called. It ain't so bad, actually! I think this was about the only place I could find MIDIs of Jean-Michel Jarre's music. It also has a lot of video game themes. Check out the music to Doom and Duke Nukem, both written by Robert Prince.
Classical MIDIs (many for SC-88) - English-language site in Japan
Strange Music Archives
- Fractal music, Mandelbrot music, Medieval music, Renaissance music, arrangements
("parodies," they say) of "Silent Night," programs for Cakewalk, and CGI scripts - another English-language site in Japan. Has MIDI, mp3, and RealAudio files.
MIDI Search Engine - Locate files by title. I searched for months for Tangerine Dream MIDIs and found nothing. Using this engine, I came up with several results in only a few minutes.
A more Complex MIDI Search Engine - This site (by an English teacher/Javascript Wizard?) has a form that uses more powerful search methods in case you can't directly find what you want via the MIDI Explorer link above.