Power of Us HUB: It took me a while to figure out that this is the Salesforce Success Community but specifically for nonprofits.
Girlforce: A Salesforce group empowering women in nonprofits.
Trailhead: An amazing training tool available for free from Salesforce. They have a short trail for nonprofits that does a good job explaining the Nonprofit Starter Pack as well as trails focusing on Admins and Developers.
YouTube Videos. Here is a great playlist of videos for nonprofits.