Papervision 3d [or pv3d] is an open source, realtime, 3d rendering engine for Flash.  It's written in Flash Actionscript, so it renders quickly [realtime], doesn't require the user to download a plugin, and is understandable to anyone with Actionscript or Javascript experience.  Since it's open source, it's both free and has amassed a huge community of artists and developers.

I'm just now getting into it, but here are a few links to get you started:

• Shark demo:  http://www.papervision3d.org/
• Showcase:  http://www.papervisionshowcase.com/
• Forum:  http://forum.papervision3d.org/
• Developers' blog:  http://dev.papervision3d.org/
• More tutorials:  http://pv3d.org/, http://pv3world.com/blog/
• Downloads:  http://code.google.com/p/papervision3d/downloads/list
• News:  http://dailypv3d.wordpress.com/, http://blog.papervision3d.org/