Actual homebrew ports I believe Quake 3 could play on a lan with a PC and possibly online if you had a valid cd key, its been years since I tested that however so i'm not 100% sure now.
It's probably not really worth it in 2018 to be honest, i'd imagine the majority of people playing online are on Xbox Live with a legit / flashed console.
Last time i checked quite a few people still played Halo 2 on Xlink Kai.