If you have the good fortune of finding one in good used condition as the company is long dead, the DataHand is the best option. It completely reversed my carpal tunnel syndrome, tenosynovitis, and tendonitis, saving me from having to undergo any sort of invasive procedures.
From the currently available commercial keyboards, your best bet is probably the Maltron followed somewhat closely by the Kinesis. If you get a "letter of medical necessity" from your physician, it is possible that some insurance companies may cover one or the other. I personally paid for the DataHand out of pocket because at the time, ergonomic entry devices, while available, were not quite as recognized as is the case now. When I was having the symptoms, I had employer covered twice-weekly chair massages focusing on the upper extremities, NSAIDS around the clock, wrist braces to force my wrists to maintain correct posture, and handeze therapeutic gloves underneath the braces to prevent chafing. After about six months or so on the DataHand, I no longer had to use the braces except periodically at night and now I don't use the braces/handeze at all. I still get some discomfort after doing a lot of typing on Maltron/Kinesis, almost instantaneous discomfort on any flat keyboard (including my Model M which I keep around as a guilty pleasure), and no discomfort whatsoever with the DataHand.
Barring the above, you could always make your own from the various open source options available today. I personally will be doing this in the near future with the lalboard.com as it is the closest analogue available to the DataHand and my DataHand is very likely not going to outlast me.
My current workstation (above) consists of a Herman-Miller Aeron with Posturefit upgrade and DataHand Pro II mounted on the arms. My elbows/forearms/palms are completely supported in proper alignment with this arrangement. For entry not requiring consistently shifting between mouse and keyboard, I use this primarily. For work requiring frequent shifting between pointing device and keyboard, I have an Ergo-Rest for both forearms with the height correctly adjusted to a Maltron M90. I split the workload when pointing by using right hand controlling movement via an ITAC Evolution Mouse-Trak trackball and left hand clicking using the buttons for the integrated trackball on the Maltron. The Ergo-Rests make a big difference as they support the entire weight of my arms leaving my hands to "float" above the Maltron and comfortably rest on the ITAC Evolution. Occasionally I will put either the Maltron, a Kinesis, or the Model M on my lap. For portable situations, I use a Kinesis in conjunction with the ITAC Evolution, though ergonomics in general tends to go out the window when going portable.
Ergonomics is very subjective and you, the end user, has to determine what combination of body posture, furniture, and input devices will work for your situation, especially over the long term.
