It took me some time to single out the source of this pop-up in the HTML source code of my blog's template, but finally I succeeded; the damned bastard spammer is the code provided by web statistics "service" Nedstat. Here is the evidence I could gathered:
- Using an option available in Firefox extension Web Developper, I check the cookies used by the pop-up window and some of the string values there correspond to strings featured in Nedstat code.
- Most important: code removed - no pop-up, code reinstalled - pop-up.
Finally, let me warn you that may be your page has this kind of problem and you did not noticed because you have blocked pop-ups in your browser settings, so you don't suffer pop-ups but may be some of your visitors who don't block pop-ups do. So I suggest you enable pop-ups for all the sites you maintain, to be sure they are pop-up free.