I use Microsoft Ergonomic 4000.
They keys take a lot to press, so I'm considering a mechanical keyboard like a rat7.
However, I feel the layout of most keyboards (rectangular) isn't adapted to a natural form of my hands (more open, spread away from the torso instead of closed in).
What's the best keyboard for long-term programming?
The two major keyboard fora, which you can use to help research and discuss what would suit you, are http://deskthority.net/ and http://geekhack.org/