I thought development was dead... apparently not, there's even a 'Next' branch with constant updates.
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Name: AssaultCube
Open-Source: Yes
Homepage: http://assault.cubers.net/
Source: https://github.com/assaultcube/AC/commits/master
Description:
AssaultCube is a FREE, multiplayer, first-person shooter game, based on the CUBE engine.
Taking place in realistic environments, with fast, arcade gameplay, it's addictive and fun!
With efficient bandwidth usage, it's low-latency and can even run over a 56Kbps connection.
It's tiny too, weighing in at a lightweight 40 MB package available for Windows, Mac and Linux.
On the correct settings, it can even run on old hardware (Pentium III and above).