Originally Posted By: MartinB
For thoroughly authentic piano and organ sounds you might want to check out physical instrument simulations such as -> Pianoteq

Pianoteq ... I am really not impressed, the piano's sound canned, a little distorted. Too much cheap sounding reverb.
OK when solo maybe but for mixing (MIDI or not) instrument stuff i used the Toontrack EZkeys Vintage piano quite often in the past.

@Rustyspoon is right, some GM stuff sounds quite good, in the past i used Hypersonic, and now HalionSonic which is 64 bit even.
The free SE version is also quite good. Plus that the pianos sound quite better than Pianoteq.