Libintroduce aims to make it easy for game developers to write games that support match-making on the Internet, and that allow game servers to be hosted behind NAT firewalls without having to enable port forwarding.
Use of a publicly available matchmaker server is necessary to make the system work, altough the traffic to this server is intended to be low, and does not include any game session data.
The first release, version 0.1, is available at http://www.speakeasy.net/~hplus/libintroduce.html. This is a pre-release, that might compile, but probably has lots of bugs, but I had to get it up there after it's been on my disk for half a year!
Contact me if you're interested in using, or contributing to, libintroduce!