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.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
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 think it was Okular. I was looking for a way of editing PDFs without ads, invasive software, or subscription based payments.
OpenFOAM. I needed powerful software to do CFD that was free, as in free beer, and found OpenFOAM. To run it on windows, the installation instructions first step was install linux on a vm then follow the linux installation instructions. I did that and started using the vm for other stuff until I found myself using the vm for most tasks, but kept using windows for gaming. To learn more, I got myself a pinebook, which replaced most of the usage of my vm. When windows decided to self destruct, I had learnt enough to install and configure arch with minimal help. Now, nothing can persuade me to go back.