Autodesk Sketchbook is a sketching and painting software that is available for Windows, Mac OS, Android an iOS devices. Autodesk has now announced that from now on the software is completely free to use. There still is a payed version that gets extended support and more features. As of Autodesk, the software will still be developed, but it is possible that not all features of the payed version will make it into the free one.
Early in it’s life cycle Sketchbook Pro was a sketching app designed for concept artists and product designers, but it evolved into a painting app with a broader audience.
SketchBook 8.6 is available for Windows 7+ (with a separate Windows 10 version) and macOS 10.12+. The mobile apps are available for Android 4.0.3+ and iOS 10.0+.
The download is free, but you need an account with Autodesk to get it, so you “pay” with at least your email.
More information on Sketchbooks official website.