<img src=http://upload.wikimedia.org/wikipedia/en/thumb/3/3d/Tsuyoshishikkarisfc.jpg/250px-Tsuyoshishikkarisfc.jpg>
With friends over, when we feel competitive and just a bit too lazy to do anything else: Tsuyoshi Shikkari Shinasai: Taisen Pazurudama (SNES/1994)... you just move the power switch on the SNES, roll out two pads and everyone can join, and even people who have little sympathy for videogames will be sucked in almost instantly with the candy coloured graphics, the mad soundscapes from Japan's last century and people going excited all around them. No discs, no gigawatt power consumption, just good clean fun for everyone.
Then there's great Amiga classics such as:
PROJECTYLE by Eldritch the Cat (Electronic Arts, 1990)
Speedball 2 (1990) - we missed this one on XBLA, so I have no clue as to whether it's any good on the 360. I'd love to be able to play Speedball 2 (or any of these old gems) without the cables or the ever-failing microswitches. Don't get me wrong - the feeling is great, but when things break, it's meanwhile become somewhat complicated to get replacement parts.
Wings of Fury (1990 Broderbund) Yeah, not the most politically correct title out there, but it's great fun after people learn how to get the plane up in the air instead of going straight for the water. It's a simple game, but it went very well during breaks of, say, Call of Duty: World at War
e-motion, vaxine, backlash - these are still amazing fun, but they really only shine on glossy 4:3 tube screens... on modern 16:9 displays, they positively look like crap and most of the effects are severely messed up and cannot be fixed, not even by turning off all the post-process trickery of modern equipment.
C=64:
Cauldron II (1986) - just bouncing around and dying is good fun!
Wizball (1987) - this game may not look like much, but colouring the world on a big screen and having the chip music blast over 7.1 is a very, very sweet experience
RanaRama (1987) - doesn't look like much, but is an addictive kafkaesque free re-interpretation of Gauntlet, where there's no barbarian, archer, mage or valkyrie, just a hopping frog. Hopping around is half the fun!
Paradroid (1985) - The game itself is insanely addictive, but what really blows today's brains is the incessant attack of strange frequencies. Therapeutical when played properly, infuriating and hopeless when played too aggressively.
All these games twenty years and older, and they are still great fun. Due to the technical infrastructure having changed quite a bit, they may look quite ugly at first, but some of this can be remedied, such as by turning down the sharpness or whatever digital trickery that really does more harm than good on old, low-resolution gaming... to get back such things as (virtual) scanlines, emulators would probably be needed, but I don't have any such rig set up... yet. It's sort of inevitable, it seems, especially after seeing how crap re-releases of classic games are handled most of the time (strange aspect ratios, eggheads, corrupted audio, speed issues, nonsensical, ugly background images or cropped 4:3 video content... etc. etc. etc...)