I got what you're looking for
If it's Safari 5.0 and later, then it definitely supports extensions:
The extension is called: Safari 5 Mouse Gestures:
just download and double click, will install automatically.
Now you need to map the middle mouse button. You can even map the right or left clicks, but it's not recommended as it might conflicts with Safari's regular functions.
Now set the middle button to execute gestures with, then combine it with: a down gesture for example (by Down Gesture I mean just moving the mouse downward). Now whenever you're in an active tab, just click anywhere on the screen and hold with middle mouse button, and drag down, it will close the tab. Much cooler than the regular middle mouse click.
You can also map the MMB + upper mouse movement (or upper gesture), and set it to Open New TAB!