There were a number of smaller libraries closer to the X11 primitives for drawing. Motif was one that i recall which powered opencde and like environments. We had used it for an industrial control system. I ended up using fluxbox as my windows manager for a while which has its own toolkit for decorating windows and drawing menus. It was quite a bit lighter than gnome and pretty much thinly wrapped XFT (fonts), and xlib for drawing, etc.
