OpenArena is by OpenArena Team and is licensed under GPLv2. BFG10K Arena mod (by WireHead Studios) was downloadable for free; its included stuff is shown here as "fair use". Screenshot captured by The Gig.
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Just decompress it (if you are using Windows, you can self-extract the file if you downloaded...
Details Screenshot from OpenArena running BFG10K Arena mod. A BFG shot is exploding. Licensing...