Best pop-up/pop-in code

Does anyone have any idea what code the most reliable (in terms of display success) for pop-ups/slide-ins/etc.

I don't use it a ton, but I have a few older sites using pop-ups with around a 50% display and I really need to get that up. The slide ins seem popular now.

Any thoughts?

I think that instant pop over is a nice one. I heard great review about it but never really tried it. Anyway if you need it I think that you can find it somewhere around the download section.