Or you could go even further back and play Double Dragon...that was co-op too
Neverwinter Nights can be played in co-op mode, but you have to do each mission individually, I think; but Dungeon Siege was full co-op..I remember playing it with Emmy a few years back.
The classic "Gauntlet" was also a co-op game..
Yea, bots, which are are literally player entities controlled by AI. It's not COOP. Not in my eyes anyway. Bots on Source is just like playing against real players with wallhacks and they are boring because they don't speak.
Synergy. Allows you to play Half-Life2, HL2 Episode One, HL2 Ep 2, HL2 Lost Coast, and some extra maps, everything on co-op or any of the disponible gamemodes you wish
It comes for free when you have at least one Source powered game... (but for playing the full campaign online (HL2 for example) you (and your friends) need to own the games you want to play, but you can always play the Synergy maps anyway and they are free ).
This mod is just amazing, it saves everything you do, even the position and the objects you were carrying on the last level, and they are brought with everything else (weapons, health, position, speed) to the next level on the game, it's amazingly synchronized. I play it so much.