Here are some popular - legal freeware - time killers:
PySOL Mahjong-like (single player) games
3D (Space) Invaders
Amanagi (vertical space shooter)
yay space invaders. yaya mahjong.
and i know its not free but audiosurf is bloody brilliant!
when i get up gimmie sum more
Nostalgia is only made worse over time, if only we could go back to a time when we werent nostalgic.