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!
Clutter is an open source (LGPLv2.1+) software library for creating fast, compelling, portable, and dynamic graphical user interfaces. It is a core part of MeeGo, and is supported by the open source community. Its development is sponsored by Intel.
Clutter uses OpenGL for rendering (and
... [More] optionally OpenGL|ES for use on mobile and embedded platforms), but wraps an easy to use, efficient, flexible API around GL's complexity.
Clutter enforces no particular user interface style, but provides a rich, generic foundation for higher-level toolkits tailored to specific needs. [Less]
libNUI is a hardware accelerated GUI framework that makes it possible to build rich multi-platform applications based on 3D rendered dynamic layouts. Interfaces are built as a composition of widgets and behaviors, and the framework handles positioning, resizing, anchoring, and texture stretching.
... [More] Objects are connected with synchronous events and delegates for mono-threaded communication, or asynchronous notifications and message queues are used for multi-threaded applications. It also supports strings (including Unicode), paths, files, data streams, fonts, threads, critical sections, audio buffer rendering, and more. [Less]
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]