I stumbled upon Goat Attack today, which is a nice GPLv3 multiplayer shooter which reminds Teeworlds a lot, but is only played with the keyboard (while Teeworlds is keyboard + mouse).
Source repo: https://github.com/goatattack/goatattack
It builds and runs well, and is quite funny. The existing servers seem more or less empty, but I've played DeathMatch and Capture the Flag with some buddies, it's fun
Upstream doesn't seem that active lately, but I've opened a good number of issues (and a couple pull requests) to improve the packaging, let's see what comes out of it. My main concern is the lack of explicit licensing information for the data (see this issue), I fear that some of the music tracks might be nonfree - will see what upstream answers.