Many older games no longer have network support since GameSpy closed, but 333networks has an Open Source replacement for the old Masterserver system.

There are other game network systems, but they all require you run some extra software. The 333networks "faster" master is the only drop-in-replacement that works in the same way as the old GameSpy.

Many games are already supported and can easily be resurrected by simply changing some lines in the game config files, such as the Unreal and Unreal Tournament games.

Other games may need the team to examine the way it is configured and then make the changes needed. Where possible contact with the game authors will me made to discuss official support. 333networks is now the official MasterServer choice for several game companies.

If you have a game in the list that needs support, it can be a good idea to visit the IRC room and chat with the team.

If you are producing a game that needs a free and independent network, perhaps 333networks could be your solution. It is distributed via Github, and can be self-hosted by clans or communities.