For me ist was FreeCAD. From there to Linux and down the Rabbithole
Free and Open Source Software
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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For me, the gateway was via palm pilot careware. My dad had a PDA when I was a kid, and he let me learn how to program it. Then I learned that there were websites to download software for it, and some of that software was "careware", ie pay only if you're able. Something clicked in my head that I could both write and access software without cost being a barrier, and that got me reading about FLOSS philosophy as I entered high school and suddenly I was dual booting ubuntu on the intel iBook I had saved up for and then it was too late for me: FLOSS had me.
Teeworlds. When I was a kid I searched up "free online multiplayer games on pc" and it actually led me to this Wikipedia article full of open source games. I tried out teeworlds and I was hooked on it and it led me to playing other open source games like cube 2 and open arena. In my head, the term open source meant "free stuff". Searching for open source stuff led me to discovering Linux and trying it though the Wubi installer and eventually moving to it a few years later.
I've used a few open source programs before studying CS without knowing what FOSS was, but the time when I really got into it and started diving deeper is probably after installing Arch Linux
WWIV
I had used plenty of open source products in the past, but the first one I truly learned the "why it's important" is home assistant. Seeing the strong community and reading more about open source projects and why it's to everyone's benefit.
We can make a far superior, safer, and community first product.
OpenTTD. Sure, I had used android, but it was the first thing I consciously knew was open source and enjoyed.