ALMOST A BUILT-IN FLATTENING SHORTCUT

I say "almost" because it works "almost" all the time. If you have a multi-layered file and want to quickly flatten it, you can usually press Shift-Command-E (PC: Shift-Control-E). The only time it doesn't work is when you have a hidden layer, because what you're pressing is the new keyboard shortcut for Merge Visible. If all your layers are visible, it flattens them, but if even one layer is hidden, it won't merge all the layers, only the visible ones. So you can use this "almost" all of the time.