SoundFonts are a technology used by some soundcards that can change the instruments used to play midi, (or change the way they sound).
Soundfonts were originally developed by Creative Labs, (makers of Soundblaster, Soundblaster Live, etc).
Soundfonts work by replacing the stock general midi soundset used to play midi on your computer. (Usually with higher quality soundsets).
You can download
free soundfonts at
http://www.sonicimplants.com/freesoundfonts.htm