Mac OS X only: Breeze is a handy window management app that lives in your Mac menubar and lets you save the current state of any window in any application. You can also assign a keyboard shortcut to the saved window state to easily activate it at any time.
Breeze saves the state of a single window — the window currently in focus — and not a specific arrangement of windows and their contents. This may seem a little basic, but it allows you to put together custom and complex arrangements very easily. You can save multiple states, assign shortcuts to each of them, and then enact those shortcuts when you need to move the windows around. You're also able to localise arrangements (and their shortcuts) to specific applications so you can reuse them while not enforcing the same saved window state in every app. If you want to do that, however, you can save a state globally as well.
While you can appreciate Breeze if you simply like organised window structures, it is probably an app that's most appreciated by neurotic people (like me). If you fall into this category, you'll really like that you can create a shortcut that always makes your windows the perfect size. You won't have to guess — they'll always be just how you want them. Sure, you'll be indulging your OCD tendencies, but at least your inner demon's cries for exactness will be quieted.
Breeze costs $US8, but it functions indefinitely for free as nagware. If you don't mind missing an occasional message (which takes no more than pressing the spacebar) you won't have to pay for it. (But you should pay for it if you like it and use it often, because supporting developers is a good thing.)