This trick doesn't get a lot of people excited when you talk about it - but if you work with a lot of open windows - it rocks!
If you have a window that is not maximzed you can grab the top bar of it with your mouse and drag it to the top portion of the screen. This will put the window in full screen mode. This is especially helpful when you're working with multiple displays. It's such a little thing - but I use it all the time. I can't tell you how many times a day I open a window on my primary display, drag it to my secondary display and then full screen it. I love it.
For those of you who compare documents you can drag the documents to the left and right side of the screens and compare two docs side by side. (You can also just use Windows+Left (or right) arrow and move the window.)
Bonus: For those of you with multiple monitors Windows+Shift+Left arrow (or right) will move your current window to the left or right monitor.