Microsoft's Natural Keyboard Elite is the ONE TO CHOOSE!

This keyboard is great.
It dramatically reduces stress placed on your wrists when typing compared to the "straight" kind of keyboards.
Keypad is "broken" in two halves which eases finding right keys for faster, more accurate typing when using a classical "touch typing" method. All keys are "concave-in," in addition 'F' & 'J' are clearly marked with raised bars which makes it easy for finger placing. Non-slippery texture has a nice feel to it, while it is still easy to clean
The "split" angle of keyboard allows for straight wrist-hand alignment, thus virtually eliminating pain from carpal tunnel syndrome and reducing fatigue. Integrated palm rest supports your palms and doesn't break off like in other models. PS/2 connector assures compatibility with almost any PC currently on the market; the keyboard can be used with optional PS/2 to USB adaptor expanding compatibility even further.
The only snag I experience when switching between this keyboard and the regular straight one is the difference in arrangement of the arrow & “Home, End, Delete, Insert, Page Up / Page Down" key sections. Highly reliable (made in 2004 it is still my keyboard of choice in the office).
Overall, this is an excellent choice for one who needs to type a lot and does not need all the fancy features.