If you want exactly the same as XNA, you should go with MonoGame. You can use your existing code (if you already started learning it) and it even works on different platforms. They're actively working on it and also adding more features.
If you want to have more features and "convenience stuff", you could go with Unity, like runonthespot mentioned. However, I'd suggest waiting until the native 2D tools are released, because working with Unity 2D Frameworks isn't that much fun, although I've only used "Orthello 2D". I heard 2D Toolkit is good, but also costs $60 or something like that.
Other than that, I don't really know of any more good C# libraries/frameworks that are kinda like XNA.