To fully exploit the capabilities of SynthFont you definitely need a good General Midi SoundFont. A GM SoundFont has presets (instruments) defined for all 128 melodic midi programs, plus at least one percussion set. Within this list you will find links both to GM SoundFonts and to SoundFonts with individual instruments.
If you know of a site you think is worth listing here, then please contact me.
For a list of SoundFonts I recommend and offer for direct download, go here.
There are more and more large SoundFonts popping up. Here's another one, 4 GB in size!. It is claimed to be SC88-Pro compatible. It has 24 bit audio, which makes it bigger than usual SoundFonts with 16 bit audio.
"Musical Artifacts is an open source web app helping musicians to find, share and preserve the artifacts they use for producing their music." Among other things you find one of the largest GM/GS SoundFonts here: the DSoundFont by Strix SoundFont Team. But you don't really need the big one - get the smaller DSoundFontV4 instead.
John Nebauer has released a Steinway Piano SoundFont from the samples provided by University of Iowa (Samples are Creative Commons Licence) as well as a nice Acoustic Guitar using the samples provided by Keith Smith.
Says Rick Simon: "I made a SoundFont that is General Midi, General Midi 2, Yamaha XG, and Roland GS compatible." ... " I have tried many SoundFonts, commercial and free, and I think it comes in favorably with higher quality samples yet keeping a smaller size for ease of use and quicker downloading. It is also compatible with virtually every midi song file available. "
This bank includes many famous sounds from the best synthesizers by Roland (D-50, Sound Canvas...), Korg (M1, X5...), Yamaha (MU, Clavinova...), Fairlight (CMI), E-MU (Emulator), Ensoniq, and many others.
Giant Soundfont 5.5: Note that you will need to download banks 1, 2, and 3 of v5.5 as well as the drumkit which is labelled v3.0. Giant soundfont is 450 MB uncompressed, the author updates it regularly.
"Don has worked to perfect this unique soundfont, and has authorized Midkar.com to share it as a Free SF for all MIDI enthusiasts. Timbres Of Heaven is Roland GS compatible. This means that there are many more instruments available than a standard GM set."
"I have made a large soundfont for orchestra with realistic (mostly studio recorded) audio instead of generic MIDI... I then mixed those into the default soundfont, so that my good ones replace what they can, but the old MIDI for the ones I didn't have are still there..."