JavaScript Alert Cancel Button
I wrote this script a while ago, but I just now made it into a Greasemonkey script, which makes it a lot more useful:
http://www.tumuski.com/2008/05/javascript-alert-cancel-button/

It's for those times when, like me, you've irresponsibly used JavaScript alerts for some quick debugging, and one of them turns out to be in a loop. This adds a Cancel button to alerts, allowing you to temporarily disable them.

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