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I've been playing xonotic and DAMN is it fun
(xonotic.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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How.does.it.compare.to open arena?
I'm not too familiar with open arena, and it's been a long time I played Xonotic (and it was not all that long), but
Additional movement, and better guidance.
The Newbie Guide has an overview of Xonotic stuff