Off the top of my head, based on my old netbook since I havent gamed on such low specs in a while:
Marathon 1/2/3 (via alephone) (these are legally free)
The original Doom 1/2
Quake
Quake 2
HL Opposing Force
HL Blue Shift
Basically, I'd expect anything from the 80s/90s up to the early 2000s to run comfortably if there's no compatability issues or there's a modern source port which isn't too demanding. I don't know if dual booting with Linux could potentially provide some performance improvements.
Mods can also potentially extend the life of a game.
Depending on your standards, maybe something like HL2 can run if you tweak the settings, but that was back on source 2006, so I don't know if that's still the case.
Of course there's also emulators, I'd expect up to the PS1 to run OK as well.