I think the rarest thing I've got is some pre-order Zelda bonus thing. Didnt even realize it was worth money. Bought a Gamecube when the prices went super low and picked up this no-box disc that included Wind Waker, OoT, Majora's Mask and like 3 other Zelda games (cant remember offhand). Got tired of the case so googled to try to print at least a case sleeve, when I found out the thing was worth like 100 bucks back then. Apparently it was a pre-order bonus for some game and was super rare....and I paid like 20 bucks for it at GameStop. One of the few times I was happy the clerks there tend to have very little gaming knowledge.
Edit: Ahh that sucks, just checked it out on Ebay and dropped price big time. When I bought it, the unopened ones were going for like 200-300 bucks and the opened/caseless ones were going for 50-100, now looks like the unopened are 100. Eh oh well, never planned on selling it anyways, just always thought it was kind of funny that the Gamestop clerks put something fairly rare like that up on the shelf for 20 bucks.