Is It Down Right Now? - Network & Admin
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
Statsignal - Statsignal is a simple and cost-effective way to monitor your website or server and publish a status page of your system.
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
OnlineOrNot - Uptime monitoring for software teams (even teams of one).