Aside from Irrlicht, the only possibilities for open source 3D with any real potential I've seen are the OgreDotNET wrapper, RealmForge, and plain Managed DirectX.
Development on the OgreDotNET wrapper seems to be a bit slow. They don't appear to be nearly as far as Irrlicht with dotNET, and it's still just a wrapper around the c++ version.
RealmForge development seems to be extremely slow, but from what I can tell, seems to have more of a pure dotNET aim. They also have some very ambitious goals which could make it difficult to get off the ground.
I have had some experience with Managed DirectX. It's fairly well documented, and there's a rather large community. Unfortunately, it is pickier with hardware and only runs on Windows systems.
That's one of the main reasons I am looking at Irrlicht. With its inclusion of DX9, DX8, and OpenGL, it has more ability to play nice on other systems.
3D in .NET - Who would've guessed!