Don't let appearances fool you, the content is strong with this one! ichillapp.com could be so easy to overlook. It's just a few pages of white text on a black background, organized behind big green buttons. It's developed by the Trauma Research Institute, and offers core concepts for getting in touch with your own natural rhythms and staying resilient in the face of life's ups and downs. It's written in plain English and is easy to use.
There's an app and a web version, virtually identical although the app has optional audio so you don't have to read it yourself. This simple feature makes it much more useful for people really trying to work on these skills at home. Make sure all other audio apps are completely closed for the audio to play. The app version also has a mood/anxiety assessment tool that's not on the web, but is easy to recreate with pen and paper.
To expand it's usefulness, I copied the text off the web into a word document that I could print out and give to clients who don't use tech at all. I also helped a client download the app during session to use at home. We're using it as a way to structure our work for a few weeks, going through one section at a time. I really love how simple it is, and even find its 1990's graphics charming. It's free and easy, bite-sized education.