OpenArena! <3 It didn't work for me (something about the v6 address not resolving, and then a stream of 'connection reset by peer' msgs in the OA console; too bad it has to be in-browser and I can't just launch my native client) but I love that this is a thing, it's one of my favorite games ever.

Edit: NES games also don't work for me, I keep seeing "Load a ROM file to play a NES game!" but others in the room are playing. Firefox on Linux, this is. Can I register a ticket somewhere or are there debugging tips I could try myself?