To "maximize" Safari (to make the window fill up the entire monitor rather than resize to fit the current content), drag the link below into your bookmark bar, then click it. Note that it only works when you have a single document open, so use it before opening additional tabs.
Longer Answer (With More Background)
Mac users know how unpredictable the "zoom" button (green "+" button in the top left-hand corner) can be. In iTunes, it toggles between a mini-player mode, and a standard display mode; in Safari, it resizes the window to fit the content in the current tab; in Mail.app, it operates just like the maximize button on Windows. Rather than being consistent across applications, the behavior of the zoom button is determined by the application developer.
I use both Firefox and Safari frequently, and I often find it annoying that the zoom button works differently across the two browsers (in Firefox, zoom maximizes the window as it would on Windows). I find that I seldom need Safari automatically resized to fit the content I'm viewing, and would much rather it maximize the window to the full size of the monitor, so I created a bookmarklet to do just that.
To use this bookmarklet, simply drag the link below into your bookmark menu:
The only limitation of the maximize bookmarklet is that it won't work if Safari has more than one tab open in the current window. Therefore, you should use it as soon as you open Safari, or open a new additional window before using it.