Genuine Soundware: They have a great rhodes simulation plug-in [ mr. ray 73], check and try a few of their leagacy [older] vsti's to see if they will work for you... free ones...
http://www.genuinesoundware.com/?a=products

LinPlug: Classic well developed plugins that are a good value with great stabilty even though they are rather expensive. I have the octopuss and two older discountinued models.. great service from staff.
http://www.linplug.com/index.htm

Algo Music: Check out some of the free synths offered for download as well as their reasonably priced main lines.
http://www.algomusic.net/index.html

Camel Audio: Makers of widely used chameleon 5000 which is on countless name artists records and albums. Some free very usable effects plugins here as well.
http://www.camelaudio.com/products.php

Algorithmic Music Sytems & Synthesizer VSTi by H.G. Fortune: Very nicely designed site with some unique synthesizer approaches... this site has lots of usable demos that actually function quite well and main products are reasonably priced.
http://www.hgf-synthesizer.de/