Elementary OS
Elementary is an open source OS based on Linux that runs on macOS and Windows, and costs as little or as much as you want. It looks VERY much like a colorful and playful version of macOS.

Elementary ships with all the basic apps, such as: browser, email, photos, calendar, text editor, terminal, music player and more. They also have an open source App Store.

Take a look at some screenshots below (click the image to imbiggify them).
Elementary OS - Browser
Elementary OS - Music
Elementary OS - Calendar
Elementary OS - File Manager

Elementary OS claims to be safe, secure, stable and collects no personal info. Nor does it display any ads. Their income is solely based on what users choose to pay. Interesting. For as little as a $1, I could see buying a copy to run on an older Mac that I’m not using anymore.