Mac OS X Lion added the option of resizing your windows from any edge, but you can do even more and that. The option and shift keys open up two new possibilities as well.
Holding down shift while resizing will lock the window’s aspect ratio and scale it evenly. This is really great when you want to maintain the shape of your window but need it to be a little bigger or smaller.
Holding down option while resizing will alter the window from both sides. Normally when you resize a window you’re just pulling at a single edge at a time. If you hold down option, your actions will be mirrored. The same thing happens when you pull on a corner, but all sides of the window react in perfect harmony.
If you want a demo, check out the video above.