One of Divvy's coolest features is you can build preset window positions and attach them to keyboard chords. So for me, cmd-shift-space brings up divvy then hitting 1 makes the app fill the window, 2 is upper left 2/3 of the screen, 3 is upper right 2/3 of the screen, etc -- I have 7 or 8 presets. Full keyboard control of window position is awesome. It's the best $13 I've ever spent.

retailmenot also claims a $20% code, so that will knock some money off. But I happily paid retail and I use it 100 times a day. It's also particularly helpful if you use an external monitor at your desk and regularly resize windows as you attach or detach the monitor.