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.
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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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
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.
WWIV
DDWRT technically came first for me, and m0n0wall, but OpenSolaris is where I really started to use it.
OpenTTD. Sure, I had used android, but it was the first thing I consciously knew was open source and enjoyed.