If you are using WordPress CMS there are a few snappy options that work very well.
Originally Posted by jakeparker
(not totally free, but will help make you more money through higher conversions)
1 - http://www.popupdomination.com/live/
2 - http://wpleadrocket.com/
1 - Great for a better user experience to get the lead conversion before they are ready to leave the page.
2 - Highly customizable to be used as a landing page or exit page.
Both offer HIGH conversions if the copy is written well.
It's free, simple, and fast.
However some feel that it is not the best user experience to force someone to stay on a page after they decided to leave.
Simply design your landing/exit pages to offer the discount or collect the lead.
Then write the exit message, and finish the exit pop-up.
This is the basic code, which should be customized to your application:
var exitsplashmessage = "Wait! Before You Go- I Want To Give You An Amazing Discount Offer! Check Out This 50% Off Discount On Your Favorite Widget";
var exitsplashpage = 'http://your-website.com/the-special-offer-or-discount-page/';
testing to find what works for you to get the highest conversions.