I have just added a new bit of software to my site www.reallyhonest.co.uk called Exitgrabber. This is similar to a popup but works when your visitors are "thinking" about leaving your site. What actually happens is a new screen appears when your visitor moves his mouse up toward the task bar. You create the new screen as a web page and use the exitgrrabber software to give you the HTML code. It's very easy to use and I have done a couple of  Google searches and there is very little reference to it in the UK and although it is a US product there is'nt much known about it there. What is mentioned though is that it is producing good results in keeping visitors for that bit longer so as to create sales. It's well worth a look plus they offer an affiliate program paying 50% per sale.