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Away3D is another 3D engine for Flash. It was derived from Papervision3D and brought new features which were not present in Papervision. Later the engine was rewritten so now it's a different engine but with similar architecture to Papervision.
Later Away3D got so advanced that some unique features from it were implemented in Papervision3D.

Currently Away3D and Papervision3D are two separate projects. Both of them are of great quality and its difficult to choose which one to use. From what I have read on the various blog posts it seams that Away3D is superior in features and functionality, Papervision3D in documentation and users base.

Some articles on PV3D, Away3D comparsion:
http://coobico.com/index.php/blog/article/3d_in_flash_papervision_away3d...
http://blog.papervision3d.org/2007/05/16/papervision3d-to-merge-away3d-f...
http://osflash.org/pipermail/osflash_osflash.org/2007-August/014001.html
Good one:
http://groups.google.com/group/away3d-dev/browse_thread/thread/d9c029574...

If I would start a 3D project now, I would consider Away3D to use. Based on the posts I read, the functionality of Away3D seems better (more functions, better results).