Lightbox
This page demonstrates the ‘Lightbox’ effect.
The opt-in form fades in after the visitor has been on the page for 7 seconds. The time can be altered to suit your preferences.
One advantage of this kind of opt-in form is that it’s very easy to add to practically any web page, without affecting the existing design. A single line of Javascript is added to the page.
This particular Lightbox opt-in form is set to display every time a visitor returns to (or refreshes) this page. It’s also possible to show it only once, or to show it a certain number of days apart.