Magnitus: So, in short, beyond Serious Sam I & II:
Unreal Gold
System Shock 2
Just be prepared to deal with glitches. System Shock 2 is a superb game but the multiplayer mode is incredibly glitchy and crashy. Unfortunately I couldn't get it working at all when I last attempted.
Saint's Row, The Third
Doom I & II
Worth noting, the classic vanilla Dooms won't give you a great multiplayer experience. It is best to look into source ports such as the multiplayer-focused Zandronum as well as GZDoom. They will bring lots of improvements to the game UI, allow a variety of multiplayer options (such as co-op survival mode, with limited lives, the ability to change item pickup behavior, etc.) and run nice on modern systems & networking protocols.
With a good source port, the Dooms are really fun games. But if that weren't enough, the Doom community has produced *incredible* custom mapsets over the past 20+ years, ones that by and far surpass the originals in visuals and gameplay. Highly recommended.
Ghost Recon
Hidden & Dangerous 2
SWAT 4
Thanks for the great feedback guys. The list, while unfortunately a small fraction of shooters, is longer than I original thought it would be.
Judge Dredd should have co-op too, although I have not tested it. The game really isn't good though, at least not in single player. Maybe it could be a little more fun to play with somebody else.
Dying Light (which HunchBluntley linked above) is definitely worth looking into, although it seems to encourage you to melee more than shoot, and from what I gathered, it doesn't have tons of gun variety. So it's not exactly a traditional zombie shooter.
HunchBluntley: Of course, it wouldn't be GOG if everything was categorized exhaustively and correctly; I definitely remember having seen some games (can't remember which ones, though) that I knew for certain had co-op, but to which GOG hadn't applied that feature tag
Yeah, that is very annoying. Others don't even have the multiplayer tag! (See Hidden & Dangerous 2)
Sometimes you know a game has or has had a multiplayer or co-op mode, but you don't know whether it's actually there and working in the GOG version until you dig around.
