I often have this problem with my brother too. I kinda want torchlight multiplayer like games, i don't know why noone ever made it... it's just Diablo with WoW graphics, oh well.

We play some Borderlands again, but in your case it wouldn't fit.

So yeah:
- Magicka (you will laugh a lot :P)
- Minecraft
- League of Legends (free DotA game)
- Starcraft I+II
- Open Transport Tycoon
- Diablo 2 (or simliar, like Baldurs Gate, Titan Quest, ..)
- Trine
- The Settlers II