The place is absolutely awash with the things :-) Almost as soon as I started researching some of the links I'd gathered I built up the following list:
And that's just a small list I could dig out, being as ruthless as possible over what is/is not a softsynth. Tons, eh? Here's what's on the excellent MAZ Sound Softsynth Mirror at:
Some list eh, and a really GREAT page on the whole range of softsynths. I've personally tried and recommend all the items on the list marked with a **, but don't take my word for it, dive in and try some for yourselves...
Here's some other good softsynth starting places...
http://www.software-technology.com - VAZ
http://www.native-instruments.de - Generator
http://www.synthzone.com/softsyn.htm - another excellent softsynth site...