I have been using the Microsoft Ergonomic Keyboard 4000 for over a year now and I love it! Unlike ordinary keyboards, the keys aren't cramped together and the soft cusion at the bottom of the keyboard makes typing feel comfortable. I bought it for about £40 at PC world (UK) and I believe its a great investment.
The keyboard comes with a couple of multimedia keys at the top such as play, volume up, volume down, keys at the top. It also comes with 5 programmable keys to launch any programs you want. There is also a "Web/search/mail" key which you can program as you wish. However, to use the programmable keys, you will have to install intellitype, Microsoft keyboard software. On a plus side, the program use very little RAM/Processor power. However, the keys of the keyboard is positioned differently from other conventional keyboards so it would take a little to get use to. However, it is designed so for comfortable typing. So far I don't have any strained wrist and the "downward slope" seagull design is way better than the upward sloping design.
In summary I belive that the Microsoft Ergonomic Keyboard 4000 is a great investment and everybody should get one!