Admin link back to website button

Hi,

I was wondering how would i echo a url from where the user was before entering the admin area.

e.g.
1. The user is on a page (lets say aboutus.html)
2. The click on the toolbar to enter the Admin Dashbaord
3. When they are done i want a button to simply to go back to the front end website last visited page e.g.(aboutus.html) – This is where they entered.

Any ideas on how?

Thanks

Nick