synths
fluidsynth
a real-time software synthesizer based on the SoundFont 2 specifications
zynaddsubfx
modernised: yoshimi
mx44
mx44, a polyphonic multichannel midi realtime software synthesizer
phasex
a [P]hase [H]armonic [A]dvanced [S]ynthesis [EX]periment
local copy: phasex.git.tar.bz2
WhySynth
a dssi softsynth plugin
supercollider
an audio synthesis programming language
wikis:
MusicTechnology swiki
sourceforge wiki
related
puredata
a "visual" audio programming system
om
is now ingen
buzz
buzz, a modular synthesizer
minicomputer
minicomputer, "a standalone Linux softwaresynthesizer for creating experimental electronic sounds as its often used in but not limited to Industrial music, IDM, EBM, Glitch, sound design and minimal electronic. It is monophonic but can produce up to 8 different sounds at the same time."
soundfonts
sfArk
a decompressor for an obscure format of soundfonts
further reading:
http://www.synthzone.com/softsyn.htm has a big list on softsynths