Of late, I've taken an interest in Open Source Software (OSS). Maybe because I don't have anything interesting to code at work!
I am just scratching the surface at the moment... trying to figure out how you go about OSS anyways... here are some links that should help:
http://www.kegel.com/academy/opensource.html
http://www.linuxchix.org/content/courses/contribute.html
http://ask.slashdot.org/article.pl?sid=07/05/10/1811251
http://producingoss.com/en/index.html
http://drnicwilliams.com/2007/06/01/8-steps-for-fixing-other-peoples-code