Microsoft just open-sourced 6502 BASIC (BASIC M6502 8K VER 1.1) from 1978. The code powered the Commodore PET, VIC-20, and C64, and underlies Applesoft BASIC on Apple II. Download it on GitHub to run, ...
If Windows is your OS of choice, consider these open-source apps. There are tons of open-source apps for just about any task. These are all free to install and use on Windows. I've been using ...