The Mechwarrior 4 mektek pack is still out there somewhere if you like big stompy robots.

Liero is pretty oldschool, but it was a fan favorite back in the day.

There are lots of old boomer/arena shooters with open source code. See Cube or old id Software games (not sure if anyone has released fully free playable versions of these).

all of them

we always had fun with WC3 custom maps, although that takes a bit of setup and idk about mac compatibility

if you've got controllers there's always MAME and a million other emulators for some good old arcade/console multiplayer, that Simpson's beat-em-up and Soul Calibur have always been a hit

the first 2 or 3 Quakes are open source now

I guess not 100% fully open source or whatever but I'm still a fan of the original 90s DOOM/DOOM II etc. and it will run on anything. I used to use Chocolate Doom which I think worked on mac? but there are other versions if you want more modern features

you just need the map files (WADs) but you can find them all over, there are free ones, custom maps, or one person can just buy the originals or download them and then give them to everyone else

Since no ones mentioned it yet, Super Tux Kart should work. Its available cross platform, and you can host LAN games if Im remembering right, and it should be fun for 30 minutes of pick up play.

I remember TeeWorlds being pretty fun

Fistful of Frags or Pirates, Vikings, and Knights 2 are both good if you know how to set up a LAN party on a source engine game