I have a bunch of old games, but there are many NES games that are still easy to get on Ebay and whatnot for an agreeable price.

For NES, I have the first Dragon Warrior, and the two gold cartridge Zelda games, but I know a considerable amount of people who have those (the Zelda games).

For SNES, I have Super Mario RPG. I don't know how rare it is, but I only know a couple people around here who have it.

For PS1, I have Lunar 2: Eternal Blue Complete (with pendant, hardcover manual, otaku figures and all that jazz). It cost me about $70.

Kind of off-topic, but I just realised that about 75% of my PS1 library consists of Squaresoft games.