Pop-up ads are nothing new. Neither are their ugly evil twin the pop-under. They have been around a while and most browsers do a decent job at blocking a lot of them. However they don't block them all because some sites legitimately use pop-up windows. If browsers blocked them all then many sites would lose part of their functionality. But you would think that most users by now have figured out that these unwanted windows that appear out of nowhere are simply blatant advertisements. But they haven't. And some pop-up and pop-under ads tailor directly to these users who are unaware of this type of spam. If you've spent any amount of time surfing the web then you have seen the ads that try to disguise themselves as system windows or an application that is telling you have viruses or something of that nature and you need to download their software to fix it. And of course there are the ads that promise to make you a millionaire with only a few hours of work.
Personally I don't use any plug-ins or add-ons that would block them because I know that some sites do use them legitimately and I don't want to miss out on any of the functionality. Anyways most modern browsers will block multiple attempts to open separate windows and give you the option to open them for that site. But sometimes this doesn't work for some reason and I still get attacked by a seemingly endless amount of windows that come out of nowhere. As a web developer, it is my duty to keep use of pop-up windows to a minimum. And if a client requests a pop-up window I politely tell them that it is a bad idea and why they should try and figure something else out. I have used them in the past - for legitimate site functionality and for advertisements. The ads that I've tried in the past don't perform well at all. I've gotten a few clicks on the ads, but none that convert. So while I may have made a few pennies for a couple of wild clicks, pop-up and pop-under windows have never gotten me any sales or leads. So if you are thinking of using pop-ups, think twice. And if you are thinking of using pop-unders, I highly suggest not to. If a person leaves a page it is because it doesn't interest them, you don't need to keep attacking them with windows that they won't see until long after they leave your site. Sure you may get a few pennies CPM but it is not worth it to annoy your visitors. If a person gets annoyed by your site it is very unlikely for them to come back. And I bet that Google and other spiders can detect them in your page source and I guarantee that they don't like it. So think of pop-up and pop-under ads like drugs, and just say no!