Apple - quick resize app to half of window on Lion

Spectacle (github, homepage) is free, and works fine for me.

There are lots and lots of similar window management apps available, the one I like to use and recommend is called Moom. Sounds a stupid name, but it's short for Move and Zoom.

Amongst various other functions (lots more, like edge snapping etc), it allows you to:

Move your mouse over the green zoom button in any window, and Moom's mouse control overlay will appear ... [allowing you to]:

• Move & Zoom to Full Screen

• Move & Zoom to Left Half

• Move & Zoom to Right Half

• Move & Zoom to Top Half

• Move & Zoom to Bottom Half

• Revert to Original Dimensions

Keyboard shortcuts are also available and customisable:

Keyboard mode allows you to do everything you can do in mouse mode, and more. It's all controlled by the Keyboard section of Moom's settings window.

Once set up, you can use the keyboard to zoom windows to full/half/quarter-screen sizes, move them (while optionally confining them to the display) around, send them to another display; grow and shrink them, center them, and revert them to their pre-Mooming dimensions.

After pressing the keyboard mode trigger key you define, an onscreen overlay appears. While the overlay is visible, you can use easy-to-remember keys (e.g., the arrow keys, plus Return, Tab, and Space) to control the frontmost window, instead of having to memorize tons of outlandish key combos.

If you want a reminder as to your defined shortcuts, you can display a cheat sheet (enabled in Moom's Settings) along with the overlay:

So essentially, yes, you can replicate the feature diretly, with the same keystroke, only with plenty more options. Moom isn;t free, but it's not particularly expensive either. Plenty more window managers are available that do similar, and even cost less. This is just my recommendation.

To emulate this from Windows, check out BetterTouchTool.