Sample Tank 2 FREE is, well, free. But it's also a demo for the full version. I like the piano. I have not purchased the full version but this is making me consider it. It can be hard on the CPU but you have a much better CPU than I.
http://www.sampletank.com/Main.html?STFreeDwn
Xpand! (I'm not excited, the '!' is part of it's name ) is only $10 From Digidesign. It's best feature IMHO is you can easily layer upto 4 Intruments to create new sounds.
http://www.digidesign.com/index.cfm?...51&itemid=4784