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.
Unicomp owns the rights to the design of “Model M keyboard”, and currently makes modern version of keyboard with this mechanism. You can buy it at: http://pckeyboard.com/.
Here's a interview with Unicomp guy Mr Muyskens npr.org.
I think it is really nice, would want it, but due to the fact that it is not ergonomic keyboard, i can't use.
Note: the buckling spring is a older technology for key tactile feedback. There are few other key switch mechanisms with good tactile feedback. If you want something with crispy feedback, look for the one called Cherry MX Blue. See: Guide to Computer Keyboard Key Switch Mechanisms.
For alternatives, see List of Keyboards with Mechanical Switches. And if you just want mechanical keyboard in standard PC keyboard form-factor and priced under $100, i recommend:Disqus