a href="http://google.com/" onclick="window.open (this.href, 'popupwindow', 'width=400,height=300,scrollbars,resizable'); return false;"

This allows a right-click to open in new window if needed, and also allows user to bookmark the pop-up. Standard javascript pop-ups without an href do not allow for this.

Link from:
http://www.youngpup.net/?request=/articles/how-to-create-popups.xml