Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The free Safari plugin Safari Stand adds this function, as well as a lot of other cool things like single window mode, automatically closing the downloads window, tab thumbnails, etc.

1. Install Safari Stand

2. Open Safari and click the View menu, then select Customize Address Bar...

3. In the window that drops down, find the new tab button and drag it to a location on your toolbar and click Done. When this new button is pressed, a new tab is created in Safari.

🙂
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.