Planet Mozilla is at http://planet.mozilla.org, and is simply an aggregation of the blogs of people associated with Mozilla software development. I’ve had it in my rss reader for about a year now, and the benefit hasn’t been in the form I expected when I added it.
For one thing, it’s truly just an aggregation of their blogs, so it’s not all about Mozilla stuff. I read about Gervase Markham’s conservative evangelical Christian theology, and other people’s trips to Spain, and new cars, and kids and all kinds of stuff.
I also learn several days beforehand when there’s going to be a new release of something, and why it’s cool. I’ve learned lots of good stuff about Canvas, and Xforms, and the processes involved in getting standards in place.
All in all, it’s been fun an educational, and not just all about Mozilla. I highly recommend it to anyone wo develops content for the web.