As I was looking for a little optin popup... I found Aweber have one. (Yes, some of you may know this... but not all! :p )
After creating your list and choosing the settings, you go to webforms. (one of the top tabs)
Go to "Create Webform" once you have done so, you go to "show more" under templates.
Then you choose "Hover Forms", Decide which template you wish to use.
Choose "Form Type" which will have popup, lightbox etc.. Choose the settings you desire! Then save it. and go to the basic settings. Choose your desired settings which will take you then to installing your webform.
Choose: "I will install my webform"
Grab the java link and then go to your WP backend.
Once inside there, go to "appearance" and then "editor"
Go to the "Footer" section and paste your link at the bottom! And voila!
Hope this helps!