Serhii Londar (Patreon) maintains a list of open-source applications available for MacOS. Unlike some similar guides, it's exhaustive rather than curated, and easy to reference in a single, well-organized HTML page.
List of awesome open source applications for macOS. This list contains a lot of native, and cross-platform apps. The main goal of this repository is to find free open source apps and start contributing. Feel free to contribute to the list, any suggestions are welcome!
I found out about Screentendo, which turns your screen into a Mario level.