Free GM Resource: A Twofer from RPG Crossing
RPG Crossing is a HUGE online community of gamers where you can run just about any type of play-by-post game. Membership is free (of course) and setting up an account is easy.
It looks like the site used to have a different name, so you may have heard about it before......
Oh, that 2nd freebie.....they have their own PDF zine called Explosive Runes. At the time of this post issues 3-19 are available for download. Even if you don't ever plan on playing in a play-by-post game, you gotta love some free zines!