Windows: Dual monitors are great for productivity. You can reference something on one monitor while working on the other, but if you find that the work on your secondary monitor requires more attention, free app SwapWin can swap your active windows instantly.
Say you've got your browser open on your secondary monitor for reference and your favourite word processor open on your primary monitor for writing. If, at a certain point, you find you need to do a bit more in-depth research (so you're focusing the majority of your attention on the browser and the word processor becomes secondary), you might experience a bit of discomfort, especially depending on how your monitors are set up. Free program SwapWin detects which windows you're working with and will swap them with a simple double-click. Of course, assigning a hotkey to the app with AutoHotkey would probably be the best way to use it.
Note that it doesn't swap the entire monitors — your primary monitor still has the taskbar on it, and it doesn't even swap every application, only the "active" ones. I still don't know how it determines which applications are the ones you're working with (maybe the window size?), but it always seemed to "just know" which windows I wanted swapped. It's nice, because it only swaps the ones you need swapped and will keep your IM client, gadgets, Windows Explorer windows and so on on the same monitor. It's neat little utility if you use your dual monitors a lot for work, so hit the link to check it out.
SwapWin is a free download for Windows only.