Mozilla Planet! That site was a big part of how I immersed myself in open source and software generally. When I was in high school I would read every post on the web site, and eventually started reading it on Google Reader.

It's where I learned about bug trackers, version control, Javascript, CSS, web browser development (obviously), and lots more. It's interesting to drop into a long-running conversation to see how professionals talk about things, and it's a big part of why I'm an open source contributor today.