Yes, that ones been around for a while and it's still annoying. My esteem for a site halves when I see it.

it's a blessing when I am grooving to grooveshark and I accidently click the close tab

so it does have it's place but it is overused

My esteem for a site halves when I see it.

Make that "reduces by 90%" for me. ;)

It's pretty useful on pages in which you input data as a reminder your data hasn't been submitted or saved, but annoying as hell for regular browsing. IMO it doesn't increase conversions, it turns people away.

There's probably some way to disable JS in your browser just for window.onbeforeunload, but I've never looked into it . . . ESC key, corner X or continue . . .

I've seen it being used on web hosts and a couple of other sites that offer an X% discount 'before you leave'. Nice touch and I agree the technique has its plus points (mistakes and things I genuinely may be interested in), but it's also often poorly implemented and/or just annoying.

I meet it most often on government sites when I click on a link to a regular dot com. In that context it makes sense; they're essentially saying "The site you're going to is solid enough for us to link to it, but do please note that it isn't the government, so you're no longer getting official information".

Unless it is justified (there are some situations where you might lose data that may require it), it's a spammy tactic and I consider any web site using this to be scammers. Now some web site that can be a bit of corporate kindergardens for many newbies (ex: Facebook, Amazon, etc) also use something similar on external links to remind people that they are venturing in the wild, and I can almost understand it to some degree if they always end up with complaints by confused visitors unrelated to their web properties, but it is still a bit of an aggressive tactic.

Seen quite a few of them usually from "get rich quick" sites very annoying unfortunately its often abused and is just another way to keep you from leaving.

I haven't seen this message box. The reason is, scripting is disabled by default as well as cookies. I only enable them for sites I trust and only for the domain I visit. So for example if your site uses ga I may enable scripting for your site but you'll never see my visit in gwt.

There are browser plugins that have to do with privacy and can improve secure browsing.