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.