I recently acquired Pianoteq after trying the demo. This virtual piano is extraordinary. Sounds great, takes no CPU, costs nothing. You have a lot of possibilities. Playing Mozart on a Pianoforte Walter is completely amazing.
I use it with my Kawai CA63.
+ Sounds great
+ Does not require a lot of CPU
+ Works on linux !
+ I would have like to have more predefined piano models
I recorded an extract of the second movement of Ravel G Concerto with the Erard model !
For further details about Pianoteq see http://www.pianoteq.com/