StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
UptimeBar - A simple OS X menu bar app for monitoring website uptime
Is it up? - A simple tool to check if a website or ip address is up or down.