While not an editor, there is an unique MIDI player available that
should be of interest to people on this group.  It is called Rondo
and is available for Mac OS X.  What is appealing about this program
is that it displays the music as a piano roll traveling over the
tracker bar and it allows changes in tempo just like a player piano.
It is a great display, but it is hard to justify spending $25 just
for that when QuickTime or iTunes plays the MIDI files just fine and
both are free.  A limited function free demo of Rondo is available.
 [ More at http://www.fracturedsoftware.com/ ]

You can download it by going to versiontracker:
http://www.versiontracker.com/php/search.php? mode=basic&action=search&str=rondo&plt%5B%5D=macosx&x=0&y=0

Also, you can use this web site to see all the MIDI players/editors
available for Windows.