Polyphone Software has released Polyphone, a free and open source SoundFont editor for Windows and Linux.
Here’s what they have to say about it:
Polyphone is a free software for editing soundfonts in format sf2. These files contain a multitude of audio samples put together and configured so as to form musical instruments that can be used by synthesizers such as fluidsynth and played using a MIDI keyboard.
The goal of Polyphone is to provide: a simple and efficient interface for creating and editing .sf2 files, available on both Linux and Windows; and tools to facilitate and automate the edition of different parameters, making it possible to handle a large amount of data.
Polyphone is available for free download via the Polyphone site (7.1MB download size, 32-bit & 64-bit standalone application for Windows & Linux).