I have on my list the open-source wow-engine and Jiglibflash. Both have impressive demos and I'd like to know what's different and which engine is better. Do you have any experience with any of these? Do you know the pros and cons when working with these physics engines?
I would recommend jiglibflash. JiglibFlash works with Away3d and the Flash 11 beta. Once Flash 11 comes out of Beta you'll be able to use the low-level GPU APIs. Install the beta and check out the demos at http://www.jiglibflash.com/blog/2011/02/27/jiglibflashfp11/
I haven't used the WOW Engine, but it looks like it is not being developed actively, as it had no updates for almost a year.