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