microsoft natural keyboard pro or elite or something like that. those are good if you are a touch typist because it relieves strain (for me, at least). if you type with two fingers, it's worthless. its more expensive than generic keyboards, but lots of people like them. (my argument for them: if you just put your hands in front of you, your hands are not parallel, they are actually at an angle to each other. regular keyboards force you to bend your wrists in painful ways. the split works to the natural contours. going back to regular keyboards hurt!) it takes a few days of getting used to.
as everyone is different, YMMV.
and as an aside, if you are really into typing comfort, go with the dvorak typing layout. its MUCH more comfortable than qwerty.