Is there ever a good reason to force opening a new browser window?

As a power web user, I know how to use my browser and it drives me nuts when web sites force me to open a new page. My mother, by contrast, just gets confused when this happens. At the end of a web session, she closes the current browser window and wonders where all these other browsers came from.

Some companies I've worked with have a policy that all external links must open in a new window, in some kind of misguided attempt to stop people leaving their site. Instead it just annoys or confuses people.

So is there ever a good, justifiable reason to open a new window?