I've been using Popup Domination on my sites. It's cool, and flexible. I also have used the ultimate footer Ad. Again, nothing wrong with it. It works, and it does what its supposed to do. I have nothing bad to say about either of these products. Then I discovered WP Subscribers plugin. This premium plugin is so versatile that it does everything but my dishes (and I think that's in beta). For example
2. Create Opt in Forms in the footer or header
3. Create footer or headers that are just a click (not an opt in form).
4. Create pop up windows that dim the background forcing the visitor to read.
5. Create a count down timer to add a sense of urgency to have people sign up (haven't investigated that).
6. Create exit pop ups (you know the “Click Cancel to stay here because we have a special gift for you” button).
7. Once you make all these forms, you can have complete control over where they show up (and even insert them into posts, widgets, etc). You can even have them show up for specific categories.
I could see using this to boost your iTunes subscriptions (add a graphic and link it right to your iTunes page), or twitter, or email, etc.
For more information got wpsubscribers.com (affiliate link)