Layers in Photoshop!
This tip is for both 10.4 Tiger and 10.5 Leopard (it probably also works on the PC version, but I don't know the shortcut!). The layers in Photoshop is really what makes this application so powerful. When you are working on a multi-layer project, you often switch the visibility on and off to get an idea of what is changing. There is a way that lets you quickly switch a bunch of layers to visible or not visible. It is done by holding down "Command" and clicking on the button that turns it off and dragging down to make them all not visible. It is somewhat hard to explain it like this. You'll get a much better idea from this video screenshot: