Shift It brings Windows 7's Aero Snap to your Mac
Windows 7 includes a whole slew of new built-in hotkeys. One set I find myself using quite a bit are the Aero Snap keys -- win + up, down, left, and right -- to quickly reposition application windows on my desktop. If you happen to use a Mac and you want the same functionality, Shift It can provide it.
Launch the app, and it sits up in your status bar waiting for hotkeys to be pressed. Shift It offers a few more zones than Aero Snap -- in addition to halves, it also handles splitting your screen in quarters. Windows can also be centered and maximized (labeled "full screen," but it doesn't actual full-screen a la your web browser).
For Shift It to work, you'll first need to head to your system preferences and then to universal access. Check off 'enable access for assistive devices,' and you're good to go!
Shift It is a free download for OsX 10.5 and 10.6.