The Linux Kernel, not just because of the end product. I find it's organizational structure and size (both in code and contributors) inspirational.

Firefox. Even if you don't use it as your main browser, the sheer amount of work Mozilla has contributed to the world is amazing.

OpenTTD. I liked the original game, and 25 years after release some devs are still actively maintaining an open source clone with support for mods.

Git. Without it, it would not just be harder working on your own source code, it would also be harder to try out other people's projects.

FZF is possibly my favorite command line tool.

Kitty has recently become my favorite terminal.

My favorite thing open source has brought forth though is a certain mindset, which in the last decade can be felt most heavily in the fact that:

1. Scientific papers with accompanying GitHub urls, especially when it comes to AI. Cutting edge research is one git clone away.

2. There are so many open hardware projects. From raspberry pi to 3d printers to laser cutters, being a "maker" suddenly became a mainstream hobby.

Yo, so does anyone here have any experience about using Windows or mac printer drivers on GNU+Linux? So far my strategy has been to try to extract a ppd file from the Windows/mac drivers. It's an epilog fusion laser printer/engraver

Most interesting bug caused - created printer driver for vietnamese diacritical marks for old matrix printer without VN support by switching to graphics mode and plotting the dots and dashes. Print speed reduced about 200x and printer shook itself to pieces by rotating the cylinder back and forth. Oh well... Ended up buying two new laser printers instead.