For those who deal with a lot of windows, especially on multiple screens, any utility that makes it easier to organize and reposition those windows quickly is a boon. I've used SizeUp for a while now, and have become very accustomed to being able to quickly snap my windows into halves or quarters of a screen and jump them from one screen (or Space) to another with keyboard commands. We've mentioned SizeUp before, as well as MercuryMover and Cinch. There's a new kid on the block, though, and it's a brilliant take on the existing ideas.
Divvy is a utility that takes advantage of both keyboard and mouse to quickly move windows into place. Where the others allowed a certain amount of control over presets, and offered a certain compromise between flexibility and speed, Divvy takes the cake by using a visual (mouse-driven) interface that also allows custom keyboard shortcuts, assigned to any part of your screen that you define.
Divvy runs in your menubar, and the main Divvy window can be called with a user-selected keyboard shortcut. It presents you with a grid representing 36 segments of your screen. Clicking and dragging a contiguous selection of those segments will snap the current window to that portion of the screen. If you want more fine-grained control, tap the command key and get quadruple the number of segments in the popup window. A trip to the preferences (the gear icon in the upper right of the window) reveals the "Shortcuts" pane, where you can assign a key or key combination to any segment of your screen. Then you can use your global shortcut to pop up the Divvy window, and press whatever key(s) you've defined to instantly activate that shortcut.
Divvy works across multiple monitors, adding a popup window to each display. If you trigger a shortcut, it will target whichever display your mouse is in at the time. I've tested it with a broad range of applications, and it's done an outstanding job. It's even conscious of "drawers," or slide-out panels on main windows, resizing the window appropriately to fit the drawer into the defined space. I'm actually using it right now in combination with SizeUp, and without any ill effects. The reason for the redundancy is mostly my own muscle memory, but I love SizeUp's ability to move windows to different screens with a single keyboard shortcut, which Divvy currently cannot do (it targets your shortcut to the screen where your cursor is, which is handy, but in many cases requires a trip to the mouse I don't need to make).
If you ever have a lot of windows on your screen, or just want to be able to quickly position a few windows into perfect sections of your screen, you definitely need to take a look at Divvy. It's free to try out, US$14.00 to own, and a single license covers all of your machines. The developers are extremely responsive and quick to fix bugs and consider enhancement suggestions, which is definitely something I take into account before buying a license. I have no complaints, and my Divvy shortcuts have already earned their spots in my mental muscle-memory library.