Which project(s) made you go “I can't believe this is open-source”?
For me personally, Zulip and Blender.
Could be due to their look and feel, performance, overall UX, or any other criteria you see fit!
For me, it's Postgres all the way. I use MS SQL Server at work and Postgres for my personal projects and I was always amazed by its maturity and feature parity with commercial databases
Ffmpeg is incredible and fundamental in a way that makes a lot of the modern media environment possible.
KDE Plasma desktop environment in particular and GNU/Linux in general. Way ahead of any closed and proprietary alternative.
Lots of developer tooling and adjacent here, no surprise. I'm gonna throw Emacs in. Amazing piece of tech.
Can't think of anything off the top of my head that would be for "general populace" though. Is open source mostly devs scratching their own itches?
IntelliJ Community is great, it has so many features making it a perfect IDE (except that it is pretty heavy)
redis, caddy, haproxy, uwebsockets.js, postgresql, lovell's sharp, zstd, zlib, many more
Rails, Hotwire, a lot of gems.