OverServer
Simple meta-server for locating other multiplayer servers
... on the player base and not a central server. OverServer tries not to violate this strength by keeping the query and response length very low to cut down on overhead from running the metaserver.
I designed OverServer with a few lessons in mind from Valve and Won's metaserver setup for Half-life. It is at least somewhat resistant to attack through a challenge-response system for incoming requests. I also wanted the tool to be very simple to "snap in" to an existing application.