This thing is really nice. You've got buttons and sliders, fully themeable through XML, with a tight immediate-mode GUI.
I know it doesn't look like much now, but I wanted to do it to see if I could start duplicating Programmer16's excellent GUI library with a lighter, cleaner interface. Next up are probably some more useful UI elements, like text boxes, list boxes and dropdown menus.
I've also been playing with learning Erlang. It seems hella powerful but esdl is total shit on OS X.