Animated_Icon -- give it a set of pixmaps and it'll display 'em.
Check_Browser -- a hold-browser that has a little checkbox next to each item that is toggled when the item is selected. Also this is a good example of how to subclass Fl_Browser_.
Progress_Bar -- the classic slider plus label.
Tb_Window -- a subclass of Fl_Gl_Window that has a trackball-like interface. Left button to pan, left+middle to zoom, middle to rotate (or left & right buttons for two-button mice). This uses the quaternion and vector routines from my util library. (You'll need OpenGL or Mesa, too.)