Here is list of website downtime checker (website monitoring service). These services will keep you informed about your websites outage and response time. You can set to receive a mail when website goes down so you can respond immediately.
Here is the list of 11 + website monitoring services which are free and cheap.
pingdom – monitor your website and servers and let you know right away when anything goes wrong.
Observu – With Observu you can monitor all your sites for free. Receive a mail when one of them goes down so you can respond immediately. There is no limit to the number of sites monitored; simply register to the left of this box by filling in the information.
Montastic – Get an email when your site goes down and Get an email when it goes back up. Montastic is completely free. The limit is set at 100.
SiteUptime – website monitoring service that checks your website at regular intervals and notifies you via email or SMS if it becomes unavailable.
Mon.itor.us – free service which Monitors 24×7 websites, network devices and more, Instantly reports your site outage and response time.
Host-Tracker – website monitoring service
Dotcom-Monitor – Website Monitoring service that ensures the reputation and uptime of your e-business by bringing together Monitoring, Reporting, Notification, Escalation and Analysis in the combination best suited to your needs.
Site24x7 – site24x7 service provides effective way to monitor the uptime and performance of your websites, online services and servers.
ServerGuard24 – provides fail-safe monitoring of your IT infrastructure, starting from simple websites up to complex business critical applications.
Service Uptime – When your website goes down, notifes you instantly via email or SMS so that you may fix a problem before it costs you money, customers or reputation. You have the ability to monitor any service every 1, 2, 3, 5, 10, 15, 30 and 60 minutes
Free Site Status – Offers Free Website Monitoring and Status Alerts and notifies you of any website downtime.