I often need to compare two windows side by side. So I wrote this AppleScript to resize the front two windows side by side, filling the screen. I have it set to run via a Quicksilver Trigger keyboard shortcut. It works in most applications.
You'll need to have "Enable access for assisstive devices" enabled under Universal Access in System Preferences for this script to work.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060105082728937