Well, for quick rectangles and stuff like that, I made a placeholder graphics folder with several rectangles of several sizes I generaly uses (16, 32, 64) and different colors.
Also recently 7Soul released some nice icons and prototyping images.
There are also other members releasing nice art that can be used for placeholder in the "Tools/ressources" forum.
It's always handy and quicker than doing and redoing rectangles each time you need something.
In the end, I managed gathering far more sprites than I actualy need/use ^^
But in need, it's always handy.