I really liked it when Valve actively started to support Linux gaming. If I remember correctly, at some point that coincided with the anticipated Steam release for Linux, they offered Portal for free with native ports for Linux and Mac OS X, and that was enough of an incentive for me to download and install it. I mainly wanted to reward them for their support for Linux gaming.
Ironically, I could very well have installed it on my MacBook, which was still running Mac OS X at the time. But I still wanted to support them for this.
Portal (the first one) is also one of the few games that I played all the way to the end, at least in these past two decades. The "your enemies have weapons, but you don't, find a way to use theirs against themselves" premise seemed quite novel to me and made for some fun, thoughtful and creative gameplay in my experience.
This must have been around 2012 or so.