Ask HN: Favourite open source game?

While there are fewer of them, there are quite a few fully open source, open content games out there, like Thrive, 0ad, Warzone2100, Endless Sky etc.

What is your favorite fully open source, open content game?

Edit: please vote on the comments people post too. Up if you like, down if you dislike, don't vote if you haven't played it or are neutral on it.

DCSS: https://crawl.develz.org

OpenTTD: https://github.com/OpenTTD/OpenTTD

I'm not sure it qualifies, as it was open sourced posthumously, but Warzone 2100 https://wz2100.net/ - was so original I've never played any other game with similar mechanics.

Battle for Wesnoth!

As an aside, does anyone have any sort of insight into how these open source games come about? Is it the force of one individual mostly or is there a place to find likeminded gamedevs for a project?