Andrew Lekashman offers a brief pictorial a history of mechanical keyboards, from adding machines to dumb terminals to Symbolics monstrosities to modern blank-key hacker totems. There was a lot of ingenious tech left by the wayside on the way to finding the perfect click.
Pictured above is one not included in the roundup, a particularly beautiful Raytheon(!) model that can be bought on eBay for $300, then sent to me.
Lekashman's tastes are grittier:
Ultrasonic I Plus
This keyboard is acoustic and operates entirely by vibration. This makes it more like a musical instrument than a workplace device. This is something that hasn't been replicated in the keyboard market since 1982. The specific principle that allows it to work is called Time Difference Of Arrival (TDOA). This is like a form of echo-location to measure which key hits the acoustic transfer bar. Whenever a switch is pressed, a metal "slapper" strikes the bar, and transducers measure the sound wave produced, which differs based on the distance of the slapper from the transducer. Typing on the keyboard is delightfully clicky and pleasantly tactile.