There are numerous ways to open a new tab in Safari. You can right click on the tab bar, use the keyboard shortcut Command + T, or use the File Menu item.
Another, less known, way is to double-click the tab bar. Double-clicking will create a new tab and highlight the address bar so you can start typing right away.
Update: Apparently (according to Simon in the comments), this is a new feature of 3.1. So if this isn’t working for you, make sure you’ve upgraded to 3.1.
Hat tip to my wife for letting me know about this one. :)