I was reading this article about the making of Torchlight and was suprised to see they used the Ogre 3D engine.
“We ended up using the Ogre 3D engine, which is open source,” says Baldree. “It doesn’t really cost anything, and it’s got really great support for older hardware. And given the size of the development team we didn’t want to spend a ton of time developing the latest pixel shader for a graphics card that only 20 or 30 people have.”
Ogre is a free open source engine with good C# bindings (if I remember) though I've heard it does suffer a little from Singletonitis. Still it's a great well test platform for development and it's open source.