Garry’s Mod. Basically a gateway drug to hobby animation, and in some ways not so far off from the modding tools used to make it.
If you’ve watched stuff like Heavy Is Dead, they’re usually made with it. Some more professional-looking stuff is instead made in “Source Filmmaker”.
This may have been my thought.
I was never so deeply in favor of the acquisition, but I did find the focus on exclusives hypocritical. Microsoft had already made many of their games available cross platform, while Sony had not at the time.