There are popups... then there are popups that work! Well, they all work to some extent but this one is truly responsive and allows precise positioning regardless of the viewing device - desktop or iPhone, the popup appears exactly where intended.
It can be customized, colours changed, permits HTML and has some really cool ways to make it appear, all from a sinple, intuitive backend.
Not least of all, the support is astoundingly fast, friendly, honest and effective. I had a minor problem that was fixed quickly and the cause (not my fault for a change) explained to me. Extensions like this are worth the money and I recommend this one unconditionally!