I picked up a copy of REZ (PS2) the other day at Movie Trading Company and am enjoying it thoroughly. I love the look of it, and the difficulty progression is pretty smooth. Plus, it's fairly original. I wouldn't want that style of shooting (press down button to lock on, let go to shoot) in all my games, but it works in this case.
That got me to thinking about how many great, semi-hard-to-find games there are for various consoles:
Odyssey2: Killer Bees! (intense, highly original game), Turtles! (excellent port)
Atari 5200: Mega Mania (better graphics than 2600 version), Bounty Bob Strikes Back (25 levels of greatness)
Saturn: Shinobi Legions (nice upgrade on classic series)
Intellivision: Diner (Intellivision exclusive sequel to BurgerTime)
I could think of numerous others, but I'd like to hear some of your (ya'll's, you guyses?) picks.