Newsletter Popup Addon GPL Overview:
Popup Extension for Newsletter adds a subscription popup with few options to set how it should show. The subscription form is exactly the same form you embed on a page or shows with the Newsletter widget.
Important update: you should consider Leads Extension which is a big improvement over the old Popup Extension.
The popup can be extended with themes, similar to the theme system of the newsletter composer. Actually there is a default theme and a more features rich theme.