I still have a really nice clean condition IBM Model M PS/2 keyboard. I've not used it for years as I never managed to get it to work with my Mac even after getting a little PS/2 socket to USB plug. Might have another go at some point as it is a superb keyboard. I'm rather tempted by one of the ones like Matthew's, though the lack of function keys and numeric keypad would take some getting used to. I'm currently using one of the flat aluminium Apple keyboards, which is a nice design item but I don't like typing on it at all. It's a bit tatty and due a replacement so time to do some research...
I still have a really nice clean condition IBM Model M PS/2 keyboard.
This is a tenkeyless version of the Model M and apparently worth $1000. Although Tenkeyless IBMs are rare and that is one of the super high end ones designed for professional and medical use.
http://www.modelfkeyboards.com/product/original-ibm-industrial-space-saving-keyboard-part-1395682/
That one really is lovely, I can fully understand why it is collectable. Mine is the standard UK layout Model M with the numeric keypad, here's an old pic:
It is the rarer version with the better quality detachable lead (not pictured) and obviously has the buckling spring clicky keys with removable keycaps (some very late ones didn't). I wish I'd been able to thieve the version with the additional AS/400 key labelling, they were really cool!
For years you couldn't give away Model Ms and you could pull one out of the office dumpster any time you wanted one. Although that's how things become collectable I suppose.
Even the original PC keyboard cost quite a chunk of money.
I'm actually horrified at the number I've personally flung into skips along with their matching PS/2 Model 50, 70, 90 etc.