As is commonly known, in Safari 4 and later, if you click on a History or Bookmark Menu item while holding the Command key pressed, the bookmark or history item opens in a new tab.
In Safari 5 (I have never tried it in version 4 but it might work) one can also open such an item in a new tab by clicking on it with the middle mouse button using a three-button mouse.
[crarko adds: I haven't tested this one with a three-button mouse. Any mouse that can be programmed to perform a Command-click should be able to have this functionality.]

