I recently downloaded a gigatorrent of NES and SNES Roms, because I've been planning to buy a netbook and the only emulation I can do on it is N64 and older, so I've been going crazy with the following games:

G.I. Joe (NES) - this thing is way ahead of its time, with amazing (for its generation) graphics, multiple character choices that actually make a difference (different characters have different speeds, height of jumps, damage, and the designs are not just pallete changes). Boss battles and game stages are also varied and challenging

Lunar Ball (NES) - it's pool. Fun.

Othello (NES) - the CPU player is fuckin cheap. I swear it's cheating.

Yeah, NES games mostly. Fun, and easy to pick up. And it only takes a couple of seconds before I get to the actual playing part. One of the things I hate with newer games is the endless barrage of developer logo screens, very few of which are skippable.