This issue really has been done to death, however, that's not to say that anyone has really come up with a satisfying answer. Here are some of the things that wise people say, and my responses to them.
Don't ever use popups of any kind - users hate them
While a certain type of mind is attracted to ideas like this, I don't think Stalinist black and white thinking will ever be attractive to the vast bulk of developers. There are lots of occassions when the optimum solution is opening a second window. I don't actually do this at the moment, but there was a time in the Style Matser product info pages when I used them to show larger screen shots when people clicked on the thumbnails. I also think they would have been a good option here in the pages that showcase the templates that will come with version 4 of Style Master.
They're great - use them whenever you want to
Actually, I don't any wise people would say this, but I'm throwing it in for the sake of completeness. I don't for example think you should use target="_blank" indiscriminately for links to pages external to your site in some sort of attempt to "trap" users in your site. As with any design choice, you need to sit down and think about the problem you're trying to solve in the broadest possible sense, and then see whether
target="_blank" is a good solution to this problem.
Here's an example of my thinking, from my recent redesign of our store pages. People on this page are at that very fragile stage of the buying process when they're about to give you their credit card details and click the "Buy Now" button. You don't want a single concern to enter their mind. You certainly don't want them to be worried about the security of the transaction. So you want to tell them that their order is secure, and do so as succintly as possible. You need to explain just how it comes to be secure. But, you also don't want them to get distracted, because as soon as they do that, their temptation to just hit the close button, save their moeny and forget all about buying your software can very quickly creep in. So, this was my problem in the broadest possible sense. I knew I had to make reference to security, Kagi, and maybe even use the Verisign site seal as Kagi does by default on all the other pages. But I didn't want people to follow this link, then get distracted, etc etc.
target="_blank" is a good solution in this situation. While of course I hate the ugly icon, I'm particualrly happy with the Verisign Seal, and how it opens a small window with information specific to the customer at that very momnent, but they can stll see the windows where they're actually making the purchase underneath. That's why I went with what I did.
But, as you'll see from the Simple Quiz on this subject, you're only half way there once you've reached this decision because you still need to decide how you're actually going to implement it. I don't want to repeat what's in the Simple Quiz, but the problem arises because
The third way
Anyway, I thought it was worth throwing in the mix. I haven't actually done anything like this myself, because I'm too lazy and I prefer to use that most disreputable method of all of including the doctype that I know my page doesn't validate to, and then waiting for some clever clogs with too much time on their hands to come along and point out to me the error of my ways. I see it as being a bit of a community service :-)