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.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
DownDetector - A simple service to tell you whether a website is down.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Outage.Report - See if your provider or web service is having an outage or it's just you. Post yours and see other's reports and complaints