Loud, clacky, spring-resistant, built like tanks -- the kind I grew up with (evolved from the IBM Model M that is still being produced today) that became the early mainstay of gamers and power users. They were big, heavy and expensive, but very reliable -- the Cooler Master QuickFire unit that I've been using is old enough to have shipped with a PS/2 adapter. Mostly found in computer retailers and expensive (>$125).

I use an IBM model M that's so old I have to use a PS/2 to USB adapter to use my 4 port HDMI/USB switch.  I have even older ones that use the 5 pin DIN connector.

BTW, I used to work for IBM and used a model M every day!