Our customer was really interested in enabling autocompletion feature for registration page. I've searched a lot, I've found a huge number of pages describing how to turn it off and (at last!) only one page about how turning it on.
All magic is hidden in AutoCompleteType property of asp:TextBox. You can read about it in details here but in a nutshell this field allows you to specify what information should be autosuggested for this particular TextBox.