What

SLIME-GTK-UI -- is GTK UI tools for SLIME.

How

M-x !describe -- show describe for object in separate window.

M-x !sexp -- show tree-view of s-expression in separate window (it is from gtk-demo), also each node can be cliked for describe window.

M-x !class -- show list of slot for class in window (from gtk-demo too). Also it posible to show subclasses (later).

M-x !function -- show tree of flow graph for function, and each function can be cliked fo describe :)