If you have a lot of windows open at once, you may find yourself doing a lot of rearranging. If you want to keep an eye on your frontmost window, though, you can move windows without focusing them.
File this into "small-but-awesome-tips-we-never-knew-about" category: It turns out that if you hold command and click on a background window's title bar, you can drag it around the screen without giving it focus. It's pretty useful if you're watching a video in your main window or just want to check on something in a background window without clicking a bunch of times to re-focus all the right windows. Check out the video for a demonstration.