Xonotic and Veloren are great.
Not a personal favorite, but many people seem to enjoy Mindustry.
In no particular order:
Online/LAN:
Xonotic: Good for online/LAN-play. UT-Style FPS.
OpenRA: Damn well good. RTS.
Warsow: Similar to Xonotic, but much faster. Damn good game. Sadly, defunct.
Sonic Robo Blast 2 Kart: If you like kart games and think they are all to easy, this is your choice.
Online/Split Screen/Couch coop:
SuperTuxKart: Damn fun, especially with a few add on tracks and good company at your place.
Hedgewars: Similar to Worms: Armageddon.
Battle for Wesnoth: Really fun once in a while. Neither the online nor the local experience is really "better".
Offline/Split Screen/Couch Coop:
Atomic Tanks: Worms on steroids.
Barbarian: Rocks. The OSS-Version is a tad bit obscure.
I didn't do VCMI, but Homm3 is one of my big local multiplayer favorites. I wait for the full inclusion of WoG before shooting it up. Also, as a young boi I really loved C-Dogs. The thing is now open source, check it out.
Barbarian: Rocks. The OSS-Version is a tad bit obscure.
can you link it? I can't find it at all.
I'm the captain of all who enjoy underground video games:
http://barbarian.1987.free.fr/indexEN.htm
Have fun :D
I prefer https://www.simutrans.com/ -- I grew up with it and OTTD feels too fiddly the way you have to lay out track one tile at a time.
I've had a blast playing Smokin' Guns : https://www.smokin-guns.org/downloads
(Veloren is pretty awesome too, in a completely different way : https://veloren.net/ )
Linley's Dungeon Crawl aka Stone Soup. The most worked and polished old school roguelike there has ever been. Just in terms of game design, it is fascinating watching this project evolve. I have never seen a project make more fearless choices, and they almost always turn out to be winners.
http://crawl.akrasiac.org:8080/
Edit: OOPS! Not really multiplayer. There are some social features on the site that hosts it allowing you to watch other people's games and heckle them and stuff.
Warfork is my favorite by far. Also, their website lists a selection of other open source games at the bottom of the page.