I Need Healing! A Guide to Why You’re Not Getting Heals

An excellent guide on the specific subject of healing, and why you aren't getting any. The guide explains some basic reasons why you may not be receiving healing, and what you can do to mitigate the current lack of healing. This guide is especially useful for DPS and tanks with high mobility, but can apply to every hero.

So You Want to Learn To Main Tank?

An introductory text, but an excellent one nonetheless. If you do not have any Overwatch tank experience, this guide is a great place to start. It introduces the basic concepts of tanking, as well as providing an overview of the tank characters. Although it is older (it does not contain any Wrecking Ball information) it is nonetheless a great "intro to tanking" style resource.

Doc’s Overwatch Support Strength/Weakness Guide

An easy-to-understand guide that covers some basic strengths and weaknesses of all the current Support heroes. This resource has the added benefit of being excellently formatted, with easy-to-parse charts if you're really into skimming.

Comprehensive Introduction to Competitive Overwatch

This twenty-page guide is an excellent introductory resource for players who are just starting with the Competitive mode. Although it is an older guide and some mechanics have changed -- the core of the guide is still extremely useful, providing a thorough walkthrough of basic concepts necessary to play the competitive mode. It's definitely a worthwhile read for newer players.

How to Improve Overall Awareness

This resource is an invaluable improvement tool, teaching you how to improve your awareness and survive longer. As with many /u/Gangsir guides, it provides well-written and detailed information in a highly organized manner.