Forte offers ultra-realistic emulation of grand pianos, using giga format sample sets. Of course, it will play any
instrument you have a compatible sample file for, but it's optimized for
pianos:

• 160-voice polyphony on suitably fast Pentium/Athlon PCs
• up to 32 layers, including pedal-up and down, and release-triggered
  samples

• high-quality reverb built in
• head-related transfer function (HRTF) for realistic, fatigue-free
  headphone listening

• realistic stereo simulation for mono sample files
• realistic pedal-down simulation for sample files without a pedal-down
  layer

Even with some of the free piano sample sets that are available, you get
surprisingly realistic sound. It's $75 US, and a free 15-day evaluation is
offered.