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...

"This page will maintain list of AI related libraries for the Ruby programming language." Some interesting stuff here, although it's all in varying degrees of maturity...