Excellent, detailed article on how Microsoft calculate TrueSkill - an algorithm for matching you to players about in your skill level. This is what is used every time you hit "game with strangers" on an XBL title, basically. Fascinating, detailed, not too challenging if you take it slow/steady - and the implementation is on github...

"Shouldn’t games be an opportunity for players to wrap their heads around counter-intuitive truths?" Yes, they should.

"On Tuesday we shipped an update that added a bunch of features / bugfixes / balancing tweaks that came out of the community's feedback. In particular, it made some changes to the underlying TF damage system, and as part of that, it modified the way critical hits are determined. We thought it might be interesting to dig a little into the change, and hopefully give you some insight into our thinking." Another cracking example of explaining game mechanics clearly and directly, to an engaged community.