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.