Previously I was promoting a software name Awesome Popup, and it is actually a great marketing tool if you are trying to get your visitor attention. Usually if you are not putting advertisements, your visitor will not get annoyed. What I did previously was to have a reminder for my visitor to join a contest.
Now you can create you own Popup with Lightbox as background. It will appear less aggresive with this smooth popup with Lightbox.
It is actually loading 2 different scripts, one for the popup and another for the background to have a nicer look.
Here is the tutorial teaching you how to d make your own smooth popup.