Good answer

I've got the NES and Genesis "Action 52s", and Cheetahmen 2.
Let's see, what else is worth mentioning?
"Recca" for Famicom (minus the box and manual because I didn't feel like forking over an extra $1000 for the privilege)
A complete set of "Sutte Hakkun" Satellaview carts
An SNES "Burn-In Test" cartridge
A 64DD setup with most of the games (minus a couple of the rarest)
From the "not that rare, just overpriced" department:
Metalstorm (NES)
Sword Master (NES)
Guardic Gaiden (Famicom)
Crusader of Centy (Genesis)
Earthbound (SNES)
EVO: Search for Eden (SNES)
Fire Emblem PoR/RD (GC/Wii)
From the "not rare in the slightest, but cosmically overpriced" department:
Mega Man 1 through 6 (NES)