I have a NOS Rebound fork, that I got from Rick the owner of the famous "Wheelsmith" bike shop in Palo Alto California when I was working there..
I guess not so rare but really hard to find a "WTB" seat post made out of a Suntour XC-pro head and a really long tube, That came with my WTB-Ti Phoenix.
I have a few different parts that are hard to find, and expensive but not rare like some of the stuff posted... I guess the rarest things I have are frames: an Ibis BowTi with Potts disk mounts and one of the prototype Ibis Ti Mojo's as ridden in a bike test in Mountain Bike magazine.
Ah, what the hell. Squirreled away in numerous cubby holes at the LBS over the past 20+ years. Liberated this afternoon @ 14:00 hrs (bailed on work early and had some time to kill). Victory is mine.