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http://nomis80.org/code/octree.html
This is my favorite. It is GPL and has it’s own homepage, so it’s meant to be used by others. It has Doxygen documentation, and the authors are taking questions.
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http://www.flipcode.com/archives/Octree_Implementation.shtml
This one is not templated. It has more comments in the code than every other I’ve seen, so that might be more useful if you are trying to find out how octrees work.