In flux… Last updated June 2017.
PowerApps Guided Learning: short tutorials and videos, started around January 2017, I believe.
Connector reference – new in May 2017, this site lists all available connectors, how to connect to them, and their triggers and actions. While this site seems to be more geared toward Flow, many of the connector reference pages include a link to PowerApps documentation on using that connector.
PowerApps Community, under the Power Users Community umbrella.
PowerApps & Flow Space, another forum, this one under the Microsoft Tech Community umbrella.
Office Blogs posts related to PowerApps
PowerApps Connector reference – This is an awesome list put together by Carlos Aguilar of Microsoft… click on a connector to see the available Actions, Triggers, and Objects (available for some connectors as of March 2017.
And beginning in March 2017, Microsoft is introducing “PowerApps Guru” blog posts… stay tuned!
Tweeters on the topic… @PowerApps, @jeffteper, @cmcnulty2000, @joleinik, @desai_darshan, @WonderLaura – or you can check out my PowerApps/Flow Twitter list! Twitter is a great place to get jump-started on a new subject, and learn more. Also try searching on the #PowerApps hashtag.