Are there any good C# Silverlight Graphics or Game engines currently? I am planning to create a game with it (either 2d or 3d) but are there already usable frameworks or should I expect to have to build everything myself?
At the moment, probably the most well known graphics engine is:
Managed GameEngine with both 2D and 3D support targetting Silverlight 2/3, Xna and OpenGL.
There is also:
Kit3D is a 3D graphics engine for Microsoft Silverlight.
As far as Game Engines go, there is:
PlayBits Engine Although PlayBits is not yet available, and will not be free.
If you're targeting both XNA and Silverlight for 2D games you can use the SilverSprite library at http://silversprite.codeplex.com and it may also be useful if you're used to a traditional game loop and draw calls.
Probably the most enhanced 3D engine for Silverlight so far is Silvermotion
It has some quite impressive demos and looks more like a high tech DirectX engine than the usual stuff and runs quite well.