When displaying your products you can now choose to have an 'other have also bought' display. If a customer has purchased multiple items then the next customer purchasing one of those items will also see a display of other products that may be of interest.
To activate the Also Bought feature enter the Set Up section of the admin and search on "xalsobought" without the quotes. Set this to Yes.
You can see below that the 'Future Desk' is the product that is being viewed, but the 'Desk Chair' is also shown at the bottom of the page because a previous customer has bought both of these.
The also bought feature is a great way to cross sell your products.
To modify the way the "Also Bought" feature works locate the FormatassociatedProducts sub routine in shopproductssub.asp and alter the html accordingly.