What are your favorite open-source games?

Widelands is a great strategy / building game. The gameplay and UI style is a niche - but that's one of the things I like about it. It's doing something different to most games.

(The gameplay is similar to Settlers 2; before that franchise changed direction.)

[edit] But the open-source game I've spent the most time playing would be OpenXcom-extended, with xpiratez. That game is truly huge.