I recently stumbled across Gwen. It's a GUI library written by the author of Gary's Mod. It's MIT licensed as any good game library should be.
The library has several things going for it.
- Large widget set
Gwen has been under development for a few years and it shows. - Easy backend renderer customization
This is a big one. The library comes with an OpenGL renderer rebuilt. But making a renderer is simple enough that you'll likely want to build a renderer specific to your game's render system. - Less filling
Gwen seems to be on the lighter side of game GUI's. No XML, no HTML renderer, just simple C++. Gwen doesn't ship with image loaders either. It expects the renderer backend to handle loading. I like this feature as it keeps the library small and my game's render system already manages textures. - Skinable
Not the first thing in your milestones but eventually you'll need to customize your GUI to your game. - C++ skinning
I'm not a web developer and find the html css centric GUIs difficult. - Input injection
This is big for games. It's always a pain to use GUI systems that want to own the input polling. - cross-platform
The library strives to be cross platform but they admit that you may find problems on less used platforms. - actively developed
Further, it's much smaller than CEGUI or MyGUI so hacking on it or submitting patches is much more practical.
