There are quite a lot of applications that support MIDI in one way or another. So I thought I'd give a brief run down of what you can expect:
MixPad from miniMusic is a MIDI player / viewer / mixer which arrived in 2007. The Pro version of MixPad (not available at the time of writing) will have recording and editing facilities as well.
NotePad again from miniMusic is a notation editor with the ability to export files to MIDI, but no MIDI import facility.
BeatPad again from miniMusic is a pattern sequencer which will export to MIDI, again, no MIDI import.
Bhajis Loops will import and export MIDI files.
Griff has some interesting MIDI control facilities using the Griff MIDI plug in, and also the MIDI network plug in. I've never got around to trying these two plug ins, but they present some interesting possibilities.
Syntrax has MIDI support.
AudioBox Micro Composer
AudioBox has MIDI import.
PDA Musician have a couple of applications that feature MIDI support. MidNote is a notation application that also has MIDI import and export, and MIDI control is a MIDI file utility that allows you to mute tracks and change instrument etc.
All in all lots of applications have MIDI support in one form or another although most are limited to simple import and export facilities.
For me the most promising will be the Pro version of MixPad when it becomes available. We'll have to wait and see.