Is It Down Right Now? - Network & Admin
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
Site Down - Site down outage and downtime reports from SiteDown.co
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
downinspector.com - Find out if a website is down for everyone or just you.
Is it up? - A simple tool to check if a website or ip address is up or down.