Good GUI library

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There is also a sticky on the alternate libraries forum listing a wide variety of free libraries.

GLUI
LibUFO
NGL
Agar

you can also use CeGUI with OpenGL

I wrote my own OpenGL+SDL library using STL for memory management, based (theoretically-speaking) on the old DirectDraw GUI tutorials on this site.

http://www.gamedev.net/reference/articles/article994.asp
http://www.gamedev.net/reference/articles/article999.asp
http://www.gamedev.net/reference/articles/article1000.asp
http://www.gamedev.net/reference/articles/article737.asp

Best of luck :)

I'll second the vote for CrazyEddiesGUI.