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IBM model M keyboard: a hardware device produced at the end of the 20th century, said by some to be the best example of keyboard design, engineering and construction.

My IBM Model M keyboard
IBM Model M keyboard

Built in 1988 and still rocking
Built in 1988

Some useful links about the Model-M:

Wikipedia : History and part numbers.
Care and feeding of an IBM Model M keyboard : Buying, disassembling and cleaning of a Model-M.
clickykeyboards : History and technical background. Also sell original IBM/Lexmark keyboards.
Unicomp : Unicomp sell brand new keyboards built around the original Lexmark technology.
IBM PS/2 Keyboard Modification : Some modifications to overcame possible compatibility problems with more modern hardware.

NOTE: the original page went offline and I'm keeping a copy of its contents on my site.
The original author is Mr. John Szybowski.

IBM PS/2 to USB : This (now archived) page describes how to upgrade a PS/2 IBM Model-M keyboard to USB.
Model-M Trivia : Various information about Model-M (and Model-F) keyboards (PCB versions, pinouts, etc.)