The following links were last verified and updated November 2017…
NOTE: In January 2018, Microsoft published a great list of resources for all levels of PowerApps users!
PowerApps Guided Learning: short tutorials and videos, started around January 2017, I believe.
Connector reference – new in May 2017, this site lists (down the left side) 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. This site includes a community blog, and an Ideas forum. November 2017 edit: The forum has now been broken out into separate discussions under “Community Boards”, which will make it much easier to post questions and find answers in the right areas!
PowerApps Ideas (UserVoice) in the Power Users Community. Search here first if you have a feature suggestion or bug report.
PowerApps & Flow Space, another forum, this one under the Microsoft Tech Community umbrella (within the SharePoint Community).
Office Blogs posts related to PowerApps (mainly prior to General Availability in the Fall of 2016).
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.
Audrie Gordon’s YouTube channel contains short tutorials as well as (nearly) weekly webinars on PowerApps topics. Audrie is Senior Program Manager for PowerApps at Microsoft.
Recorded sessions from the September 2017 Microsoft Ignite conference, related to PowerApps.
Tweeters on the topic… @PowerApps, @jeffteper, @ArtsyPowerApper, @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.