Simple enough. I've gotta MT-32 (a couple actually), and am wondering if anybody's heard of a way to use one with an MSX2+ (or earlier). I doubt that I would when I have a X68000, but I'd like to know.

As for MIDI interfaces, there's a number of options actually:

1 - the (hard to get & expensive) Bit2 MIDI interface
2 - the Philips Music Module has a MIDI interface
3 - FAC MIDI interface, made & sold by the dutch demo group FAC.
4 - JoyMIDI. A DIY MIDI interface, connected to the joystick port.

#1 comes with its own software package, MIDISaurus, and also contains a number of BASIC extensions, IIRC.
There's a couple of wannabe-sequencers, MIDI replayers and trackers that can use either #1 or #2 (and occasionally #3).
I can't remember seeing any software for #4 :-/

The Ultimate MSX FAQ and Hans Otten's pages are definitive resources to scoop for more info...

5. MidiSaurus interface. (Not same as Bit2 interface)
6. Yamaha SFG-01 or SFG-05 with UNC-01