http://youtu.be/eqhZebtA-SU
This is a cute plugin that works right out of the box. Easy install and easy setup. You can see an example here on my blog. You can use it for anything you like. You can set it up to work exactly how you like it to appear on your blog.
Did I say it’s free? Oh yes.
Just go here to download it. Or go to plugins in your admin. Just type (WordPress popup) and install and activate.