From the TiMidity++ home page on SourceForge:
TiMidity++ is a software synthesizer. It can play MIDI files by converting them into PCM waveform data; give it a MIDI data along with digital instrument data files, then it synthesizes them in real-time, and plays. It can not only play sounds, but also can save the generated waveforms into hard disks as various audio file formats.
One good use for TiMidity is to convert MOD music files (remember those?) into AIFF files, which can then be converted into AAC or MP3 tracks from within iTunes. An uninstallation script is provided and is installed into /usr/share/timidity/uninstall-timidity.sh.
This software is released under the GNU General Public License (version 2), and I have bundled it into a Mac OS X installer package. Please note that I am not the author, and thus cannot provide much more than rudimentary technical support, except in the matters of installation—but please feel free to contact me with any problems that you might have.
Click here to download TiMidity++.