For many applications, simply option-clicking the green button will "maximize" all the open windows of that application. Of course, maximize behaves differently in some applications. For example, option-clicking the green button in Safari makes all the open pages taller, but not wider. That's by design in Safari, and I rather like it. If you come from Windows, however, you'll be mortified that the window does not occupy the entire screen. In Firefox, it does indeed maximize to fill the screen. It's a matter of preference, but the key point: option-click will max all open windows of that application. Some are "smarter" than others.
As our last trick, try option-clicking the green button Calculator. It toggles between the expanded, scientific calculator to the programmer's calculator to regular calculator. Neat!