If you are using Google Chrome in Mac OS X, you will notice that clicking the green (maximize) button will only expand the browser window vertically. It doesn’t expand horizontally to fill up the screen.
To get it to expand horizontally and fill up the screen, you just have to press “Shift” while clicking the green button. The browser will automagically expand vertically and horizontally.
Note: this only applies to Google Chrome in Mac OS X. It will not work for any other application.