Looks like they do it with javascript + YUI which is pretty cool: http://showmetheheaders.com/static/showme.js http://showmetheheaders.com/static/showme.js You

Looks like they do it with javascript + YUI which is pretty cool: http://showmetheheaders.com/static/showme.js http://showmetheheaders.com/static/showme.js You can do it with curl too for all those who don't know curl -I example.com works with multiple domains as well