Open Scene Graph

Open Scene Graph is great if you want to get started with a well written scene graph and don't want to meddle too low in Open GL. I am not a rendering expert myself, all I can say is that it is of very profesional quality and meets my rendering needs. There are lots of examples and if you download the source you will get the source for those examples. I can't really comment on the performance as I haven't thoroughly compared with other scene graphs.

I don't put 5 stars because of the lack of teaching material. I downloaded the reference guide but it's just a doxygen generated document that does not contain enough explanations in my opinion. The tutorials are not enough to quickly become efficient at using this otherwise great scene graph. I would love to see a more active community around it and I feel that more teaching material would soften the learning curve and attract many more users.

I think this is the best among the free

OSG is very powerful, with well designed API, and as such does a great job of rendering games or similar applications. It's not very friendly to beginners due to being very general and making use of advanced design patterns, but that's the price for a professional engine. I've encountered a few bugs here and there, but overall my experience is very positive and I now prefer OSG over other engines such as Ogre3D.