Blender is a contender for the best go-to example in the history of open-source software. It's easy for any layperson to understand what it is (unlike, say, Linux), and it's genuinely better than the for-profit competition in a lot of aspects. There's nothing else like it.
It's been growing for decades, and its community is the best.
I'm really not in that space so I only get to watch from the sidelines, but I've always been in awe of Blender.
I'm going to have to say OpenStreetMap, it's always going to have a place in my heart.