Safari has a nice little feature that allows you to move tabs between browser windows. Just grab any tab in a window by click on it and holding the mouse down and you can then drag the tab to any other Safari window. When you do this, the it will remove the tab from the original window and create a new tab in the window it was dragged to.

If you hold down the command key when you are dragging the safari tab, instead of moving the tab into an existing open safari window, it will create an entirely new safari window and open this tab in it.