I have a few suggestions for you based on my experience:
UT2004 Probably the best FPS here on GOG with enough content in the base game to last ages and some good mods like killing floor (KFMod 2.5, not to be confused with the retail version)
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http://www.gog.com/game/unreal_tournament_2004_ece Medal of honour war chest another FPS that isn't as popular as ut2004 and there's no bots in the base game but some modders have made a coop mod which is still very WIP:
http://www.moddb.com/mods/medal-of-honor-cooperative-gameplay I faintly remember seeing some bots somewhere around. They were half finished but still did the job.
Store page:
http://www.gog.com/game/medal_of_honor_allied_assault_war_chest Soldiers: Heroes of WW2 RTS with a direct control mechanic, All missions from the campaigns + bonus missions could be played in coop. If you have played the later men of war series then this one will be similar. It's not for everyone because there is some micromanagement but I am sure a demo is available somewhere if you wanted to give it a try.
Store page:
http://www.gog.com/game/soldiers_heroes_of_world_war_ii Stronghold Crusader Fairly unique medieval RTS, popular at many LAN parties, good range of AI difficulty with different personalities and a fair level of customisation when setting up a server. GOG version offers both the HD patched version and the 'extreme' version which seems to be almost universally disliked due to the addition of weird powers, still a top game though.
Store page:
http://www.gog.com/game/stronghold_crusader Conquest: Frontier wars Classical era RTS like the old command an conquer series, but set in space. Unique features include multiple system maps, which in layman's terms means small maps with wormholes which act as choke-points. An old favourite of mine, and a demo exits somewhere
Store page:
http://www.gog.com/game/conquest_frontier_wars Signal Ops Indie First person puzzler, You play a nameless commander in a bunker somewhere in a dystopian land, commanding a team of specialists through some monitors. Signal ops is a hard game to describe and I think it fits up to 3 players max in coop, which definitely makes the experience a bit easier. I haven't played this for a while after I got stuck on a later level, but there have been at least two patches since then, one of which added several new difficulty levels. I don't know of any demo, so you would have to look on Youtube gameplay footage. there is some funny dialogue though and the unique artstyle reminds me of old oil paintings.
Store page: I have a few suggestions for you based on my experience:
IL2 Sturmovik-1946 This is without a doubt the best WW2 flight sim of all time. It is said there are over 200 flyable player aircraft. I have never counted but there seems to be an almost endless list. During my LAN sessions I hosted a dynamic campaign which I played with my brother and uncle on and off for around a month. Difficulty is fully scalable from arcade to realistic, on a per option basis making the game playable with anyone from an absolute novice to a Sim veteran. There are a lot of playable campaigns, some static (One pre-defined outcome) some dynamic (War can go back and forth). I used an external mission generator (IL2 DCG) to customise the campaign with units out of the timeframe. Excellent if you are a sim fan. note that I have the retail version in this case, but there shouldn't be a difference. also worth noting, its one of those easy to pickup, hard to master games, but don't let that put you off, making a landing on a carrier is tricky but very rewarding.
Store page:
http://www.gog.com/game/il_2_sturmovik_1946 These are just the games I could think of that have LAN and are on GOG that I have played. When I look for a multiplayer title it has to have LAN and either COOP or bots (Official or unofficial). I hope this information helps you, finding a game with LAN these days can be a real challenge.
