A free library providing windowing and widgets for graphics APIs / engines where such functionality is not natively available, or severely lacking. The library is object orientated, written in C++, and targeted at games developers who should be spending their time creating great games, not building GUI sub-systems!
Titanium Xna Game Engine is a Xna 3.1 based game engine written in c#. Complex scenes can be build in just few lines of code. The developer is free to combine TXna features and standard Xna techniques without any limitations.
libRocket (http://librocket.com) is a renderer agnostic XHTML/CSS renderer, useful for creating UIs for games and the like. Hopefully makes assembling a game UI a breeze.
Advantages of v8rocket:
Renderer agnostic, input agnostic, allows a fully dynamic layout, that can grow/shrink etc. as needed, uses existing layout technology - standardized XHTML/CSS to describe layout, allows easy "theming" via CSS, or even changing the XHTML, without having to recompile, allows delegation of UI work to developers skilled in web programming. [Less]
Manages forms in D3D. Good for creating an interactive GUI in Direct3D. Managed by Jetamay. This will soon be distributed with the GUI Manager classes, which is essentially an interface to ease the process of managing multiple forms, and sometime next week we'll have a form creator to create forms
... [More], save them and load them using a newly added method to the form class. [Less]