I’ve been spending a lot of time (maybe way too much) with Twitter lately (you can find me there as timberry) so I feel like at least one of the good things about that is suggestions such as idealist.org, which I got from a Twitter friend and seems like a really good recommendation for people looking into nonprofits or social media.
That site seems to have very good basic information on setting up and running a nonprofit, on a wide variety of topics ranging from legal to fundraising, marketing, etc.