I'm looking for some good co-op LAN games on GOG. Does anyone have any suggestions?
I'm into RPGs, but am happy to try others!

SWAT 4 isn't on GOG, but it should be, eventually. The SWAT games make some great CO-OP. The only problem is your friend tasering you for fun.

Not on GOG, but i got me "Men of war" on a sale and you can play the single player campaign on co-op. It's pretty good and fun, though I'm playing it on easy because I know the game is pretty tough.
UT 2K4. No plot, just shooting and killing pixels, you can arrange teams so every human ends up in a team and bots on the other, but it's more fun shooting at your friends.

UT99 is great on LAN But its not as good if you only have a 1 or 2 other people playing.

Heroes of Might and Magic 3 is great for some coop lan against AI. Its got light rpg elements in it as well as your hero can level up and gain new items.

Die by the Sword is very fun when playing co-op. Hazardous - the one and only modder left in the DBTS scene - has created an editor that lets you customize your enemies inside a battlefield arena, so you and a friend can take on a whole bunch of AI based on what you've chosen.

Since "Men of War" was mentioned, you can try the earlier "Soldiers: Heroes of World War 2".

Swat 4
Anything Diablo styled.
Warcraft 3 - Survival maps :)
Splinter Cell - Conviction.
Neverwinter nights 1&2.
Overlord 2 (Thou its not co-op specific content)

Troll.

Do not let ubisofts incompetence in the way of a great co-op game :)
Oh and Maybe Optimus holds some answers?
Edit: I just love GoG's reply system sometimes!

Baldur's Gate & Icewind Dale series

Another vote for this. Sacred has a whole MP mode dedicated to co-op play, and it's a RPG (even if an Action-RPG).