From a composers point of view, it would be nice to have a piece of software which models a grand piano and covers the complete range of expressions which make a piano a piano. For instance, if you hit the same note harder (with higher velocity), the sound should not only be louder, but also sound more aggressive. Also using the sustain pedal has a significant influence on the sound (and cannot be simply modelled as not releasing the notes that are already pressed. Finally, and maybe hardest to model, different notes inter act with each other (one string putting another into resonance).
So, dear lazyweb, I do know commercial products for windows that deliver high quality piano sound, either as huge sample sets, or via physical modelling. But are there comparable high quality pianos which are open source?