OGRE

I've been using OGRE for a while now and it is superb.

It runs on numerous architectures, has great documentation and has a full time developer who maintains it all.

One of the great things about this project is of course the community around OGRE. They have created numerous plugins, libraries and all sorts of things which make writing a game from scratch even easier.

Another great selling point of OGRE is that the rendering is abstracted so you can use OpenGL, DirectX or if you're porting your game to some weird alien technology you can write your own plugin.