Polyphone is an open-source soundfont editor, hosted on Sourceforge, and made by Davy Triponney. It is available for Windows, Mac OS X and Linux (It can be compiled for different platforms too).
No one currently uses it, due to the new open soundfont format, the SFZ.
Polyphone can edit:
And can read (without the ability to edit them, even if it can convert to .sf2):