Scroll down your library, and post the games you could play; by yourself, in a small group (2-4), or all together(ish).

Independently:
Deus Ex
Skyrim / Oblivion
Supreme Commander
GMod
HL1
Metro 2033
Orcs Must Die!
Portal / P2 / P Prelude
Simcity 4
Wargame
World In Conflict
TF2

Small group:
L4D2
Portal 2
TF2
Wargame?

Larger Group:
TF2