Thanks for the info, i like CEGUI, although it seems a little inflexible. Does anyone know if something like NGUI for Unity exists, but open-source and OpenGL/SDL?
There is a project called BGUI for Blender (Game Engine). Blender has a library called bgl which is a Python wrapper for OpenGL. However, I believe that the latest versions of the library support the open source opengl bindings. I'm not sure what language you're using, but Python is pretty (relatively speaking) easy to implement into most environments if you're careful.
It supports animations, but not particles. You'd need to implement them yourself. It may not be the best choice, but take a look!