Your funeral....
Sword of the Stars: 4x game with pretty ridiculous scale. It's pretty easy to quit a game in progress and come back to it later.
Terraria: Like Minecraft, but 2D and with stats, bosses, more weapons and enemies...
Source Engine mods. Synergy is a good one for co-op, while the Hidden is fun if you want a little co-op and a little versus.
Blood Bowl: Competitive only, but it's fun. Doubly so if either of you are into Warhammer.
Torchlight 1 or 2: Haven't touched this much, but it seems to be a bit like Diablo.
The Civilization series: Another 4x game, but a little more mundane.
Vindictus: Action MMO that's pretty fun.
Garry's Mod: Sandbox is always fun, but you can also play things like terrorist hunt or zombie panic.
Serious Sam: The only thing better than gunning down hordes of enemies by yourself is gunning down larger hordes with a buddy.
Portal 2: Co-op puzzle solving as quirky robots. Fun, but short.
Team Fortress/TF2: Shooting people.
Left 4 Dead/L4D2: Shooting zombies.
And that's just my Steam list off the top of my head. Next we have:
Mari0: Mario and Portal mash-up.
Neverwinter Nights: D&D-based RPG. You can download one of the dozens of user-created campaigns and have a blast.
Cortex Command: If you have a good computer and a large monitor, you can play this split-screen and obliterate clone bodies by the dozen. Great fun.
Dawn of War series: I recommend Dark Crusade, but they're all fun.
Metal Slug: Run-and-gun. Get the anthology and you'll have something like seven games in one!
Jamestown: Vertical bullet-hell shooter. Short, but the harder difficulties (which you'll need to play to unlock later levels) can be brutal.
Whew...I think that's everything I can offer at the moment.