As with all things these days, this is an ever-evolving list… Last updated November 2017.
Microsoft Flow site – The Learn tab includes links to Documentation, Support, Community, Blog, and the ability to give feedback.
Microsoft Flow documentation – As of November 2017 (at least that’s when I noticed the change), the Learn>Documentation link on the main Flow website takes you to a Docs site. It’s likely more complete and better organized now, but be aware that it takes you away from the Flow site.
Guided Learning – new in November 2016, content being added. [November 2017: updated URL to the Docs link]
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.
Connector reference – new in May 2017, this site lists all available connectors in the left pane, how to connect to them, and their triggers and actions.
Microsoft Flow Community, under the Power Users Community umbrella. Includes a Forum and a place to post Ideas. November 2017 edit: The forum has recently been broken out into separate discussions under “Community Boards”, which makes it easier to post questions and find answers in the right areas!
PowerApps & Flow Space, another forum, this one under the Microsoft Tech Community umbrella (within the SharePoint Community)..
Flow Ideas (UserVoice) in the Power Users Community. Search here first if you have a feature suggestion or bug report.
Workflow Definition Language reference – This is for LogicApps, but in particular the Function reference applies to writing “Expressions” in Flow (i.e. when you select Expression rather than Dynamic Content to fill a field). Thanks to Daniel Christian for the heads-up on this reference!
Recorded sessions from the September 2017 Microsoft Ignite conference, related to Microsoft Flow.
Office Blogs posts related to Microsoft Flow (this was mainly only used until Flow’s General Availability in October 2016).
Flow Tweeters: @MicrosoftFlow, @jeffteper, @PNWAdventureGuy, @cmcnulty2000, @merwanhade, @iscsus – or you can check out my PowerApps/Flow Twitter list! Also try searching on the #MicrosoftFlow hashtag.