As with all things these days, this is an ever-evolving list… Last updated June 2017.
Microsoft Flow site – The Learn tab includes links to Documentation, Support, Community, Blog, and the ability to give feedback. The Reference section in the Documentation doesn’t really contain much in the way of reference material, but I see that the Release Notes were updated as of the October 31 2016 General Availability release.
Guided Learning – new in November 2016, content being added.
Video gallery – new in May 2017. Videos from webinars and members of the Flow community.
Product Updates – Get the latest info on new functionality and supported services.
Microsoft Flow Community, under the Power Users Community umbrella. Includes a Forum and a place to post Ideas. Navigation leaves something to be desired.
PowerApps & Flow Space, another forum, this one under the Microsoft Tech Community umbrella.
Flow Ideas (UserVoice) in the Power Users Community.
Office Blogs posts related to Microsoft Flow (this was mainly used until Flow’s General Availability in October 2016).