Eight players, cross-platform
An update to Duke Nukem 3D: Megaton Edition has been released on Steam which adds deathmatch and cooperative gameplay modes for up to eight players online, across PC, Mac, and Linux platforms.
There is a lot about Duke Nukem 3D that I find disinteresting, but it's one of the few deathmatch-style games I have genuinely fond memories of playing. The weapons were fantastic and silly, which made the whole thing so much fun that I could forget about how terrible I was at playing it. I would not have bought the Megaton Edition without this feature, but now I'm seriously considering it.