ShadowSweeper turns shadows for windows and menus off or on, bringing a simple, clean, flat look to your screen.
Just launch ShadowSweeper to disable shadows, and launch again to bring them back. ShadowSweeper is great when placing windows side by side seamlessly, with no shadow overlap.
Although ShadowSweeper is a nifty tweak, it does have some known limitations, such as the fact that it isn’t compatible with OS X 10.11 El Capitan (it doesn’t look likely it’ll get fixed) and the shadow that appears under the menu bar is still present.
Still, for a tiny tweaking tool it this is a fair price to pay to have a bright, clean looking screen.