Soundfont resources - freepats/documentation GitHub Wiki
Freesound
Collaborative database of Creative Commons licensed sounds
Philharmonia Orchestra Sound Samples
Creative Commons licensed sound samples. Please note that their web site lists an additional condition that makes the samples non-free.
MuseScore Soundfont links
SoundFonts recommended by the MuseScore project
PlopGrizzly
A collection of public domain soundfonts and instrument/sound effect recordings.
University of Iowa Music Instrument Samples
Freely available for use without restriction.
Ethan's Original Soundfonts
Royalty free soundfonts created by Ethan Winer.
OLPC Free sound samples
Links to free sound sample resources.
Titanic Soundfont
High quality MIDI soundfont by Luke Sena. Please note that, while it is labeled as "public domain", the included LICENSE file contradicts it and reveals that it is actually non-free.