Divvy ($14) is the most useful utility I’ve added to my Mac in quite some time. Unless you're truly a one-thing-at-a-time wonder, you probably find yourself flipping between and resizing overlapping windows constantly.
Divvy takes the hassle out of window management by giving you a very clever and intuitive interface for “snapping” windows to a grid.
Just click anywhere in or on the window you want to size, launch Divvy by clicking the menu bar icon, and drag your mouse across the squares you want the window to occupy. Repeat for other windows.
You can even customize the number of squares in the grid.
To me, it feels like Divvy makes my 13” MacBook Pro screen bigger because I can effortlessly utilize every square inch of it. My most common use is aligning Notational Velocity or TextEdit in a narrow column next to Safari or Preview so that I can refer to things as I write.
Love, love, love it.