IBM Model M keyboard, Unicomp
Model M keyboard, made famous in early 1990s with IBM's PS/2 personal computer.
I used it for a few months in 1990. Click-clack click-clack, but a real joy to use.
What people love about this keyboard is its superior tactile feedback. The key switches are based on a mechanism known as buckling spring. When you put pressure on a key, after a certain point, =click!=, it sucks you down. That means, you get a clean, precise, feel whether the key is pressed. A drawback is that it is rather loud. The sound came from the spring hitting the key's inner wall. It's crispy and precise.
Many modern, rubber dome based keyboards are also loud, and makes ugly “squeak”, “clunk”, “thud” sounds, due to bad acoustic housing of the keys and plastics rubbing each other.
Where to Buy
Unicomp owns the rights to the design of “Model M keyboard”, and currently makes modern version of keyboard with this mechanism.
Here's a interview with Unicomp guy Mr Muyskens http://www.npr.org/templates/story/story.php?storyId=100076874.
IBM Model F keyboard
Model F comes before Model M.
Model F is even better, because it uses electrostatic capacitance switch. Model M is a cost-saving replacement.
The buckling spring is a old tech. There are few other key switch mechanisms with good tactile feedback. If you want something with very crispy tactile bump feel, look for keyboard with switch called Cherry MX Blue.