Ah the memories!

I can tell you quite a few that I enjoy playing with my girl:

* Icewind Dale 1 & 2
* Baldurs Gate 1 & 2
* Heroes of Might and Magic 2-5
* Age of Wonders 2 (including Shadow Magic)
* Dawn of War 2
* Alien Syndrome
* Disciples 2 (and all expansions)
* Etherlords

How about Payday: The Heist? It's supposed to be on sale on Steam now.

Nice necro there!

Well since this is necroed, I recommend SWAT 4. I found it more fun than Raven Shield, having played quite a bit of co-op with my bro, especially when trying to coordinate movements and deciding who should be giving commands. Shame more people aren't playing it, because it's really one of the best co-op games I've ever played.

I also found that NWN MP campaigns are brilliant. It's like playing DnD ... With your PC being DM :D

Persistent worlds are the best, if you have some good GMs running about. I had a lot of fun with GMs spawning random stuff in Richterm's Retreat servers. There was this dull dungeon that a friend and I were running through, and suddenly this 'NPC ghost' spawned, gave us a random quest, and spawned a freaking demon in the middle of a crypt meant for low level players. We all died lol.

Terraria
Baldur's Gate 1+2
Lego games
Portal 2

That's what I can think of for now.