From Jeff: Okay, guys, I'm kind of the "godfather" of this one so I'm going to hustle it through the process unless you have strong objections, in which case hopefully I can answer them.
Glicko2 is the head-to-head competition ranking system developed and placed into open-source by professor Mark Glickman. You can read all about it here: http://math.bu.edu/people/mg/glicko/
It is currently the preferred ranking system of the US Chess Federation and has been adopted by many other sports and even some onlien game sites. It is unqiue from most approaches to rankings that have come before in that it is a much mroe rigoursly mathematical approach. It treats the rankings of the players as "statistical unknonws" and through statistical analysis of head to head wins and losses it attempts to reduce the level of "unknowness" of everyone's ratings.
Its gotten a good deal of praise for being a system that seems to "feel" much more correct then many that went before it and it solves a lot of problems in previous systems. (There's all sorts of details on that in Prof. Glickman's papers at the above link.)
The biggest practical problem with it today for use in coding projects is that, although the algorithym itself is open and fully described by Prof. Glickman, there is no open-source reference implementation in code.
I've gotten permission from him to do an open-source implementation and they guy who is REALLY doing all the work already has an alpha done in J2SE that passes Prof. Glickman's test cases. (I can't call it a reference because its not "official" in that way, but we are striving to make it as correct as possible).
I am planning on using this for a project in Sun but we thought it would be a good thing to share with the community in general, ergo this project.