If your game needs advanced GUI capabilities, CeGui# might just hit the nail on the head for you. Marketese aside, this is a seriously good GUI library with Buttons, ListBoxes, Scrollbars, ProgressBars, Sliders, ComboBoxes and more.
Being a port of the well known CeGui C++ library, it is not bound to a single graphics API but can instead be easily extended to run on a multitude of environments. Out of the box, it already provides extensions to be used with: XNA Framework 1.0, Managed DirectX 1.0, SDL.NET and Tao.OpenGL.
The only real problem for XNA developers might be that it does not yet work on the Xbox 360 due to several restrictions in the functionality of the .NET Compact Framework 2.0 that XNA applications have to build against on the Xbox 360.