I’m running windows on my Mac. It probably took me 30min to an hour to figure it out and get it working. Not bad compared to the 4-6 hours we spent setting it up to play Halo 1 online back in like 2001-2002.
I assume this is some kind of VPN. You can play online with pretty much any VPN solution, and there are lots of those for OS X, GNU+Linux, and Windows. There are probably some for less used operating systems like Amiga OS and MorphOS too.
Hamachi was popular when I was playing around with console games and lan-only or procured (pirated) PC games. It supports OS X and Windows (Don’t know about GNU+Linux or BSD). Will probably require some configuration to make the VPN be usable for your entire LAN and not just your own PC.