Terraria

BIT.TRIP RUNNER :D or another indie platformer

Dungeon Defenders

Mmm, my GF didn't like it very much, but you could try Terraria. Let's see....lower end computer.....Mmmmmm, yeah, that's about all I can think of.

Well, if her computer can handle source, you could try the Left 4 Dead games if she's into zombies. I know my GF has a blast killing more zombies than me every single time XD

Hero Academy

Define low-end.

Left 4 Dead should work even on lower end PCs (unless you mean REALLY lower end). That was the first one my girlfriend and I played together.

Mutual depression and angst is fun too. Free to play as well. Have fun with that LDR. Have so much fun.

Portal 2! Torchlight 2 when it comes out!

No one talks about killing floor! It needs low requirements and its fun to play in coop mode (yes, I'm a girl and I love zombie games haha)

PUZZLE QUEST!!