Free GM Resource: G+
Most of the people I've already posted links to their work before so I'll focus instead on just a couple of communities:
RPG: Kind of a no-brainer, but there are some good discussions and sub communities.
Gaming on the Cheap: Again, what a no-brainer for a blog like this. Some great ideas floating around on there!
The Library of Gaming Maps: A veritable treasure trove of maps.
Order of the d30: I've just been on a d30 kick lately, probably because of all the free stuff that Richard LeBlanc has been doing last month (and before/since...just last month was crazy...)
There are more I could recommend, and even more I personally want to check out, but this is just a quartet to get you started. The cool thing is that as you join more communities, G+ does a good job of recommending even more communities that fit in with your other groups. Just click on the little arrow beside your home tab (top left of your G+ screen) and choose "communities".
I see a few I need to be checking out myself......