Don't annoy your visitors with pop-up windows.
Nobody likes popups. Here again, the only reason a site would have popups is because the site owner is thinking of his/her own interests rather than the readers. We all know that when we're browsing we hate popups, but suddenly when we switch hats and become the site owner, we lose our ability to see through the users' eyes. So let's remember to put ourselves in their shoes. Which of these reactions to popups is a visitor is more likely to have?
(a) "A popup window, oh goody! I love sites with popups! I will make certain to bookmark this site and visit often. I will also certainly click the ad or links in the popup because I love them so much."
(b) "@#&$! Whoever made this website obviously has no respect for me as a visitor. When I leave here I will never come back."