I remember having a good time on Dungeon Siege. We'd support each other, run to save our each other, QQ at the bosses together, share the loot and then take our time apart to chill in the towns we passed through. Had a great bond with the other players.
No I don't want an MMO, but more of a single player game that happens to have co-op. Skyrim online would be a fine example (if only it were playable beyond seeing naked prisoners running around).
I get 404 when I try that. Besides I was looking for a way to get only PC results and then further filter out shooters and articles for headphones and such.
Baldur's Gate 2. If you are into CRPG, awesome journey, especially if you and your friend are into forgotten realms and such. Though it is not fully action packed.