Since it was quite depressing to create this thread about DRM on GOG, because I felt it necessary, I'll cheer myself up by making a list of games that still are 100% DRM-free on all parts, including multiplayer.
Yes, DRM-free multiplayer is possible! The 'but multiplayer requires online regristration of some form' excuse, that is often toted by apologists, is false. So here are the games that offer DRM-free multiplayer, either as LAN or Direct-IP (personal dedicated server). Note that many LAN games can also be played online via Hamachi or some other form of virtual LAN.

Since I hardly play any multiplayer, please help me out extending this list with good examples of games that don't require Galaxy or any online registration. They can allow online registration to some proprietary server (like Battlenet), as long as it is not required and the games offers DRM-free alternatives for multiplayer.

[In alphabetical order]

8-bit Armies (LAN)
Act of War (LAN)
Age of Civilization II (LAN only)
Age of Wonders (original): (LAN, hotseat and play-by-email)
Age of Wonders Shadow Magic (LAN, hotseat)
AI War 1 & 2 (direct IP)
American Conquest ( LAN )
Anno Series (Anno 1701 direct IP, 1602 direct IP on older machines, 1401 and, as far as I know, the others: LAN)
ARMA: Cold War Assault (LAN and Direct IP)
ARMA Gold (LAN and Direct IP, but with a unique serial key check - so not entirely DRM free)
Baldur's Gate 1&2 (LAN and direct IP for classic, direct IP for Enhanced)
Barony (direct IP and LAN Co-op)
Battle Isle: Incubation (LAN?, direct ip?, hotseat)
Blood Fresh Supply (direct IP)
Brothers in Arms Earned in Blood ( LAN )
Brothers in Arms: Road to Hill 30 ( LAN )
Brothers in Arms Hell's Highway (LAN)
Call of Juarez (LAN, internet via Doomseeker)
Call of Juarez Bound in Blood (LAN)
Civilization IV (LAN and direct IP)
Constructor Classic (IPX)
Crime Cities (LAN)
Crypt of the NecroDancer: SYNCHRONY (LAN, direct IP)
Dark Reign ( LAN )
Darkstone ( direct IP )
Deadlick II (TCP-IP)
Delta Force ( LAN )
Descent 1+2 (LAN and online)
Deus Ex (LAN and Direct IP)
Diablo+Hellfire (LAN)
Disciples II: Rise of Elves/Gallean Return (LAN and internet TCP/IP, hotseat)
Divinity Original Sin (LAN and direct IP)
Doom I+II (LAN)
Doom III (with a trick. Check the top rated review)
Dungeons and dragons Dragonshard (LAN)
Dusk
Empire Earth (LAN, direct IP with workaround)
Empire Earth II (LAN and Direct IP, but with a unique serial key check - so not entirely DRM free)
Evil Islands (LAN)
Fallen Haven: Liberation Day (LAN/WAN)
Factorio (direct IP, LAN, dedicated server)
Falcon 4.0 (LAN)
Fantasy Wars ( LAN )
Far Cry 2(LAN)
Flatout 1 & 2
Ghost Recon
Grim Dawn (LAN)
Hammerwatch (Direct IP)
Heretic: Shadow of the Serpent Riders (LAN)
HeXen: Beyond Heretic (LAN)
HeXen: Deathkings of the Dark Citadel (LAN)
HeXen II (LAN)
Icewind Dale 1&2 (LAN and direct IP for classic, direct IP for Enhanced)
Jagged Alliance: Deadly Games - (TCP-IP by using tunneling option from Dosbox on IPX feature in the game)
Jazz Jackrabbit 2: Secret Files (LAN, with JJ2+ also via internet, split screen)
M.A.X. (TCP-IP via IPX Dosbox, hotseat)
Master of Orion 2 (TCP-IP via IPX Dosbox, hotseat)
Medal of Honor Allied Assault and Pacific Assault (Direct IP and LAN)
Mobile Forces (LAN and Direct IP)
Neverwinter Nights Enhanced Edition - LAN or Internet Direct IP.
Old World ( Hotseat and LAN )
OpenTTD (direct IP)
Overload (LAN, direct IP)
Painkiller (LAN)
Powerslave (DOS) (TCP/IP LAN)
Project Zomboid (Direct IP)
Quake I + II
Quake III (LAN and direct IP)
Quake IV (LAN)
Rainbow Six ( LAN )
Red Faction (LAN and Direct IP)
Return to Castle Wolfenstein (LAN and Direct IP)
Risk of Rain(Direct IP)
Sacred 1 & 2 gold (LAN)
Saints Row 2 (LAN)
Saints Row Gat Out of Hell (LAN)
Saints Row The Third - The Full Package (LAN, only in the original, the remaster removed LAN and only left DRMed multiplayer in)
Scrapland (LAN, direct IP, dedicated Servers)
Screencheat (LAN, Hotseat)
Secrets of Grindea (direct IP can be toggled)
Serious Sam: First Encounter/Second Encounter
Sins of a Solar Empire(Direct IP)
Sir You are being Hunted (LAN)
Soldier of Fortune II (LAN and direct IP)
Space Empires IV (TCP-IP, PBEM, hot-seat)
Starbound
Stardew Valley (LAN)
Star Trek: Away Team (TCP/IP and LAN)
Star Trek: Starfleet Command III (TCP/IP and LAN)
Star Trek: Bridge Commander (TCP/IP and LAN)
Star Trek: Elite Force II (TCP/IP and LAN)
Star Trek: Voyager - Elite Force (TCP/IP and LAN)
Star Wars - Jedi Knight II (LAN and Direct IP)
Star Wars - Jedi Academy (LAN and Direct IP)
Star Wars - Republic Commando (LAN and Direct IP)
Streets of Rogue (Hotseat, LAN, even though the product page says that Galaxy is required)
Sudden Strike ( LAN \ IP )
Sudden Strike 2 (LAN \ IP )
Sudden Strike 4 (LAN )
SWAT 3 ( LAN )
Swat 4 (LAN, direct IP)
Terraria (dedicated server)
Titan Quest (LAN)
The Wheel of Time (LAN and Direct IP, although it requires dedicated servers)
The Planet Crafter ( LAN/direct IP)
Theme Hospital (IPX)
Torchlight 2 (LAN)
Trine 1+2+4 (direct IP and LAN (unsupported but possible), don't know about 3)
Unepic
Unreal (LAN, dedicated server, direct IP)
Unreal Tournament 99 (LAN and direct)
Unreal Tournament 2004 (LAN and direct)
Van Helsing final cut (LAN)
War for the Overworld (LAN and direct IP)
Warlord Battlecry
Warcraft II (LAN)
Warhammer 40k Gladius: Relics of War (Direct IP, hotseat)
Wasteland 3 (direct IP)
Wildermyth (direct IP)
World in Conflict (LAN and Direct IP)
Worms Armageddon (LAN, direct IP)
XIII classic with the 1.4.2.1 patch (dedicated server)


... to be continued ...

Same PC multiplayer only: (Hotseat, Split-Screen, Shared Screen etc. But no online or LAN)
Antihero (head-to-head hotseat)
Axis & Allies 1942 Online ( Hotseat )
Battle Chef Brigade (split screen)
Bonkies
Broforce
Cat Quest 2
Caveblazers
Cook, Serve, Delicious! 2!! (2-player split-screen co-op)
Crawl
Cuphead
Death Road to Canada
Emperor of the Fading Suns
Epic Pinball Collection
Fist Puncher
Fort Triumph
Hellpoint (Split screen)
Hot brass
Inkulinati (Hotseat)
JYDGE
Judge Dredd - Dredd vs Death ( Split -Screen )
Lovers in a Dangerous Spacetime
Missionforce: Cyberstorm
Moon Hunters
Neon Chrome (shared screen)
Oh...Sir!! The Insult Simulator
Outward
Pixeljunk Shooter
Police Stories
Relic Hunters Zero: Remix
Shadow Empire
Songs of Conquest (Hot seat)
Spelunky
Talisman: Digital Edition
Telepath Tactics
THE KING OF FIGHTERS 2002
Tonight We Riot
Tooth and Tail (Split screen)
Towerfall: Ascension
Trash Sailors
Tyrian 2000
Wizard of Legend
Worms Forts: Under Siege
X-Morph: Defense Complete Edition

I was notified, that there is already a list of older titles with single computer multiplayer.

For a list of games where DRM-free multiplayer can be added via mods or external tools, look here.

Ghost Recon was playable in LAN at least. Stardew Valley too. Terraria. Starbound. A friend told he played Hidden & Dangerous 2 in LAN, I didn't test it myself.

Civilization IV has LAN and Direct IP multiplayer available.

Games where DRM-free multiplayer can be added via mods or external tools

- Caveblazers (moddable with Caveblazers Together)
- Elder Scrolls III - Morrowind (moddable with TES3MP)
- Fallout: New Vegas (moddable with NV:Multiplayer)
- Hollow Knight (moddable with HKMP)
- Jagged Alliance 2 (moddable with Jagged Alliance 1.13)
- Kerbal Space Program (moddable with Luna Mod or DMP)
- Nox (moddable with OpenNox)
- Nuclear Throne (moddable with Nuclear Throne Together)
- Postal 2 (moddable with Sven Co-op)
- Rimworld (moddable with Rimworld Multiplayer)
- Rollercoaster Tycoon 2 (moddable with OpenRCT2)
- Settlers 2 (moddable with Return to the Roots)
- Spelunky (moddable with Frozlunky)
- Tomb Raider 2 (moddable with Tomb-Raider-II-Multiplayer-Mod)
- Undertale (moddable with Undertale Together)

What about Divinity:Original Sin? I have it and I haven't tried the multiplayer, but it seems to have direct IP and LAN options. Perhaps someone could confirm if it is DRM-free (one way or the other)?

Btw, I have favorited both of your recent posts - they are extremely helpful. Bravo!

I think you should make a clear list with denotation on the side that states whether it's IP based, LAN or builtin lobby,

UT99 still has working online and LAN support.
UT2K4 also.
Rune does also.
Solider of Fortune...

Are these three sold on GOG? I don't see them, but that might be due to regional locking.

I'll try to include whether the DRM-free multiplayer is LAN or direct IP, as far as I know it. Feel free to help out and fill in the blanks for games, where you know what kind of multiplayer they offer.

The UT games are indexed in germany i think, so they won't show up.

OK. Thanks. I'll include them.

Warhammer 40k Gladius: Relics of War (Direct IP)
Dusk
Serious Sam: First Encounter/Second Encounter
Warcraft II (LAN)
Battle Isle: Incubation (but not sure if it's LAN, direct ip or both)
Fallen Haven: Liberation Day (LAN/WAN)
Jazz Jackrabbit 2: Secret Files (LAN, with JJ2+ also via internet)
Disciples II: Rise of Elves/Gallean Return - LAN and internet TCP/IP)

Age of Wonders Shadow Magic include some options but never tried them t know if they actually work.

Doom 1 and 2 work online with Doom Seeker

https://doomseeker.drdteam.org/

I only play online with ppl I know, so we tend to play a lot of older games. Some of us have not even left 1998 :) So I can help you populate the list with all of GOG's games that work without Galaxy because everyone I play with is on Linux.

Thanks.

Can anyone confirm that the LAN-mode works and if the internet multiplayer is DRM-free or proprietary server based?

quite depressing you say? ;)

sorry to ask again in this thread too, but are you counting multiplayer keys (I guess they aren't technically "CD keys" like I may have said previously) as DRM here or not?

It should count IMO. That's yet another artificial restriction.