ManiacTools (http://www.maniactools.com) has released MIDI to MP3
Converter for Mac OS X (10.5 and higher). The US$29.95 app lets you
convert MIDI files into into WAV or MP3. By using "SoundFonts," it
can purportedly surpass the capabilities of a particular sound card,
producing tunes that sound better, compared to the computer's usual
output.

Several SoundFonts can be used simultaneously, so the user is able to
assign individual SoundFonts to each instrument instead of using
ready-made General MIDI sets. In other words, the program turns a
usual home computer into a powerful synthesizer with a potentially
unlimited number of instrument samples, according to the folks at
ManiacTools.

Using SoundFonts, MIDI to MP3 Converter will convert a five-minute
MIDI file into MP3 in a few seconds. The program has a built-in ID3
tag editor and can supply converted files with important information,
like title, artist etc. It will accept MIDIs that are dragged and
dropped into its window. Multilingual packages are included, MIDI to
MP3 Converter will display its text labels and messages according to
the system language. A demo version is available for download.