Well the best one ever was Star Craft, I'm sure 2 is a great LAN game... oh wait.

Well, Diablo 2 was great fun on LAN soon enough D3 will... crap again!

I'm done.

Titan Quest is pretty fun on a LAN if you like ARPGs. I can also recommend Dawn of War: The Dark Crusade, which will likely run on a pretty mediocre PC these days and is a blast with a lot of folks.

The best LAN shooter is still UT2004.

Doom/Doom II. Dominated office LANs back in the day.

DoTA.

Starcraft.

Unreal Tournament.

Wolfenstein

The four classic games at any LAN party!

Only one of those is on GoG though. :p

Available on GOG?

Jagged Alliance: Deadly Games, with IPX enabled in DosBOXes is fun as hell.

Jagged Alliance 2 with the 1.13 mod supports LAN/WAN multiplayer, though, the project is currently in closed beta, due to be released soon, as in, a month ago, or so. ;)

For some blasting fun, Descents are bloody damn awesome.

Unreal Tournament, Duke Nukem 3D, no explanation needed. ;)

Grrrrrr... :D

CS Source, CoD4 Last Man Standing, UT2004 Last Man Standing, Warcraft 3.

Starcraft 2! :p

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EDIT: Bah, missed the GOG part... Well, UT2004 Last Man Standing multiplayer mode. :p

If and only if you have a good internet connection at the LAN party location.

Soldat (www.soldat.pl) is the one I have had most fun with at LAN.

Heroes of Might and Magic 3 Complete. If you want a LAN game that isn't twitch (so not a shooter or RTS) you can't do better. You and a buddy or two against five or six computer opponents on a good two level map on the highest difficulty? Excellent ^_^

Can't go wrong with Heroes of Might and Magic 3. Battle all your friends or team up against Bots and dominate the map! Pick a favorite map or have it Generate a whole new one just for you! Endless hours of fun with that.

Although Duke Nukem 3D is entertaining for a random Free for all death match of sillyness.

Fallout Tactics

... but you've to know more than the basics and it's better to play with close friends ....

You need to pick a game that you are new to. Same for your friends.

I've only played a few GOG available games on LAN, and will now review some of them as LAN games.

Fallout Tactics:
This can be fun, but in my experience (or because of the people I played with) it quickly turned into a silliness fest with miniguns and explosive dogs. The original game cover says there's a co-op mode, but sadly there isn't one. The multiplayer is squad versus stuff, you can choose it to be either turn based or real time. You can make all the characters in your squad (or choose premade ones) and choose their equipment. In my experience it's best to avoid the high damage weapons, because there's something shady with the network code and it can end up so that on someone's computer the minigun dealt 5000 damage and killed everyone, while on someone else's computer it only dealt 16 damage and everyone's alive, after which the other team gets taken out by ghosts that you can't shoot back at (happened so often we had to stop using advanced weaponry). <also, let it be mentioned that I love this game>

Icewind Dale:
This one is great fun if you're looking for an adventure with your friends. It's co-operative, and proper team work feels good (occasional screw ups can be funny too). Being sort of campaign driven, playing it properly takes time. Roleplaying-ish and not too fast paced.

Commandos:
Co-operative, "no one dare screw up" tactical(or realtime puzzle?) multiplayer. When the teamwork/tactics work, it's great. The rest of the time it can get frustrating if the team spirit isn't full of friendship magic. Not necessarily the best choice.

UT2004:
OH YEAH BABY! But people can get annoyed if someone's way better than the rest.

Serious Sams:
Both co-operative and competitive multiplayer modes available. Great fun, and can be played on split screen (useful if you're playing a turn based hotseat game on another computer and it's that warm & small kind of a lan party or if the number of available computers is lacking).

Heroes of Might & Magic 2/3/4:
These are all great. Turn based strategy pleasure. They take some time (quick combat cuts that but drains a lot of the fun out of it) and need patience. The way it was played in the LAN parties I attended was that the games were in hotseat mode on one computer and while someone had their turn, others played other games on different computers/consoles (I've only attended small LAN parties with close friends). 3 is the best of the series, but 2 has some special magic to it (might just be nostalgia talking).

Sadly, I haven't played other GOG available games in LAN parties, but outside that selection I recommend: Half-Life 1 (with mods like Sven co-op, Counter Strike, Firearms against bots <dunno if they're even available anymore>), Diablo 2: LoD, Warcraft 3: TFT, Starcraft, C&C: Generals, 2D Worms games, Triplane Turmoil, Jump 'n' Bump, Liero, One Must Fall 2097...

Sacrifice
Interesting mix of strategy and 3rd person combat.

Earth 2150
Good RTS.

Serious Sam 1&2
Excellent co-op FPS.

Unreal Tournament 2004
A good FPS with quite a few good mods available.

Descent series
3D Flight Sim/FPS. If you're anything like me you'll go insane playing this, because there is no up or down or left or right ;)

Giants Citizen Kabuto
Fun 3rd person combat with very different factions, but only if you have plenty of players.

Postal 2
Funny FPS with silly weapons.

Stronghold
Building castles and sieging the opponents castle.

Empire Earth series
Like the Age of Empires series, a decent RTS.

Not all of the available games, but a decent selection I hope.

I'd enjoy any good coop game, for example System Shock 2.