Open source software is created with the intention that it’s shared with others, and with the side effect that it’s nearly always free of charge. There are many amazing titles available for the Mac. In fact, there’s a strong chance you’re already using some. One of the Mac App Store’s top titles, The Unarchiver, is open source, for example.
The following two sites provide extremely good lists of Mac open source software:
I consider myself pretty savvy when it comes to open source software, thanks to my background writing professionally about such things, but there are several titles in those lists that I’d never heard of, and can’t wait to explore. Take a look. It’ll be well worth the effort.