Theres a ton of good stuff on the forge, but the problem is that so much of it is opaque, and you need to spend a shit ton of time studying it and sifting through long buried archives to actually understand it.
Robin's Laws of Good Gamemastering is a must read. I don't think its available for free on the internet, but its worth paying for.
Play Unsafe by Graham Walmsley is a book about bringing improv techniques into roleplaying. Lots of great advice whether you are a GM or a player.
There's probably a dozen blogs with excellent advice, but I'll have to hunt down the vital posts.