Spotted via MacWorld, you can open a new tab in Safari 3.1 by simply double clicking in an open space in the Safari window. Let me demonstrate:

Safari New Tab

See the arrow in my picture of the open space near a tab? Double click and you got a new tab. So if you are out of a keyboard, then that is how to do it.

I was hoping for this feature for a while. I know saft plugin did it, but I didn't want to add saft to Safari.