A excellent, ergonomically designed keyboard is the Kinesis's Contoured Keyboard, from http://www.kinesis-ergo.com/.
There are several quality designs that went into this keyboard. I explain each item below. Note: the design of Kinesis seems to be originated from the Maltron Keyboard. Maltron was there first.
Note that the modifier keys and other major keys such as {Space, Enter ↵, ⌦ Delete, Control, Alt, ⌫ Backspace}, are under the most powerful finger the thumb.
The ⌫ Backspace key is one of the most frequently used key, yet on a conventional keyboard it is situated at the far upper right corner. Mechanical typewriters don't have that key. It's grafted to computer keyboard later on.
The modifier keys for today's computer keyboards such as the {Ctrl, Alt, ❖ Win} are aligned on the lower left and right corners of the keyboard. Effectively, only the Alt is somewhat easy to use because it's near the thumb. The ❖ Win key requires the thumb to curve far inward to press it. The Ctrl key, cannot be press comfortably using a finger. (A technique of pressing the Ctrl key is to use the palm. 〔☛ How to Avoid the Emacs Pinky Problem〕)
The Kinesis keyboard solved this problem by moving these modifier keys right under one of the most powerful finger the thumb. So, moving the thumb to various positions, the user can hold down any of the modifier key, and other fingers of either hand can easily press the other keys to be modified with.
Microsoft also came up with a keyboard that moved the ⌫ Backspace to thumb. The Microsoft Sculpt Comfort Keyboard. However, it's pretty bad otherwise.
Note that key columns (⁖ 1 Q A Z) are laid out straight, so that your fingers moves in a straight line, not slanted as in conventional keyboards.
The reason traditional keyboards have staggered columns is due to inheritance from mechanical typewriters, see Keyboard Hardware Design Flaws.
Note the bowl shaped surface. When your hands rest on the keyboard, your wrists don't bend up. Your fingers naturally dip in and rest on the keys.
The Microsoft Natural Ergonomic Keyboard 4000, introduced in 2005, used the same ergonomic principle, by tilting the palm side of the keyboard higher.
Kinesis uses mechanical key switch. It's either Cherry MX Brown or Cherry MX Red. 〔☛ Guide to Computer Keyboard Key Switch Mechanisms〕 That means, the key press has better tactile feedback, easier on the fingers, and lasts some hundreds times longer than common rubber dome switches.
Here's the detailed layout diagram.
The ❖ Win (Windows Logo) key or the Mac's ⌘ Cmd key can be set to one of the {Alt, AltGr, Ctrl}, or other keys, because the keys can be re-programmed.
Nothing can be perfect. I think Kinesis is a superb keyboard, but here's what i think are flaws.
The function keys F1 to F12 became tiny rubber keys. Instead of normal key with good tactile sensation, now becomes rubbish. Hard to press and hard to know if it registered. For detail about this problem, see: Keyboard Shortcut Design: Dedicated keys, Special Buttons, Extra Keys.
The arrangement of F keys is now a contiguous row, instead of 3 blocks of 4 keys each. The left row has F1 to F8. The contiguous design makes it difficult to find the key without looking at the keyboard, especially for keys in the middle of the row.
The Esc key is now part of the rubber key in the F key row. The Esc key is important in many applications. Now it being a special rubber key, makes it very bad.
Many key combos with Ctrl and Alt keys now require 2 hands, because these 2 modifiers are placed further from the thumb. For example: 【Ctrl+c】 (copy), 【Alt+Tab ↹】 (switch window). They are much difficult with Kinesis than a normal Microsoft ergonomic keyboard. Apps such as Emacs, Second Life, Blender, or even just in Windows 〔☛ Microsoft Windows Keyboard Shortcuts〕, require a lot such key combos.
Also, many of 3D modeling app require using the mouse while a combination of Alt, Ctrl, ⇧ Shift held down. It may be any 2 of them at the same time. This will be difficult with Kinesis.
Following are some illustrated criticism from a Amazon reviewer Tim Q.
Not enough keys. For a programable keyboard, it should have a lot special keys that people can assign them for dedicated purposes. F1 to F12 is not enough. They are often already used up by key-intensive applications such as emacs, 3D modeling apps.
For example, if you listen to music from your computer often, you'll miss the Play/Stop, Sound Level, Next/Previous song, keys.
The bowl shaped surface makes it Inconvenient for casual use of the keyboard. Even for a touch typist, you are not always in a typing intensive situation. Sometimes you are watching a movie on your computer, or just need to press a few keys while reading in a browser. The Kinesis requires you to put both of your hands in position if you just want to press a few keys.
I think Kinesis is one of the best, along with Maltron and Truly Ergonomic Keyboard. The Truly Ergonomic Keyboard is my current favorite.
I'm also a heavy emacs user for the past 10 years. If you have RSI problem, i also recommend Dvorak Layout, and use ErgoEmacs Keybinding in emacs.
See also other Fancy Ergonomic Keyboards.