Windows: Sometimes you have more windows on your screen than you can deal with at once, even when you're using a good window manager. Peek Through makes any window temporarily transparent with a hotkey, then lets you click through to the window below it.
Peek Through is a simple app that sits in your system tray and waits for your custom hotkey. When you press it, the current window becomes transparent, and (if you've turned on the click through setting) you can click through to the window below. When you're done, just press your hotkey again and your window will become opaque.
Note that the app has a few quirks. If you use the click-through feature, you can sometimes run into a problem where you accidentally lose focus on your main window, and you can't just click on it to bring it back in focus (because clicks fall through to whatever is behind). Usually, you can just click on the program in the taskbar to bring it back into focus before pressing your hotkey again. Still, it can be useful in situations where your desktop is hidden under layers and layers of windows. Hit the link to try it out.
Peek Through is a free download for Windows only.