<a href="#" onClick="history.go(-1);return true;">Send Me Back A Page!</a>
This tells the browser to go back one page. That's it! No mater what the page is before it, it will go back. The only problem happens when that page happens to be the first that you pull up. When that happens you'll get a browser window pop up and tell you "Do you really want to close this page?" since there was no page to go back to, by the logic of your browser that is.