Ditto on String Theory, also look for plugins by EVM. They have some awesome old plugins of individual samples such as 2 choir plugins, a string plugin, flute, grand piano, etc... Good Stuff if you can find them, the Grand Piano and other cool stuff can be had at EVM
http://www.evmsynths.com/free.htm
Land of Cockaigne: Cool Stuff, Go Get It (Seriously
the Phibes elctric organ plugin is one of the creepiest sounding organs you'll ever hear).
http://www.funkelectric.com/~cockaigne/
Mik Sybrandts "GTG JP 7A" Rocks!!! (and any of his others for that matter).
http://miksybrandt.panicnow.net/downloads.htm
And for the abosolute best free Mini Moog and Arp2600 emulations go right straight to
http://glenstegner.com/softsynths.html
Check out Sampletank and Sonic Synth (Both have free demos with some cool downloadable patches:
http://www.sampletank.com/Main.html?STFreeDwn
http://www.soniksynth.com/Main.html?prod_SS
And last but not least navigate to the VST Instruments portian of
http://www.soundfonts.it/ and grab ORGANized Trio and Mr Ray (Hammond B3 and electric piano plugins respectively).
As far as KVR goes, do like LFO said, but I usually leave the "Unreleased" plugins option enabled as often you are getting the very latest plugins (Often Beta, but usually quite stable) from companies with established track records, and they will eventually no longer be free, so get while you can.
You can also search by manufaturer on that page too (Dropdown list at the top of the search page), so go check them out.