Update

Added two new monitoring services for my website so that I can get immediate alerts when my site goes down. Apart from this I also get a weekly and monthly report showing me the site uptime stats. Some things you have to look out for while subscribing to these services are:

• Monitoring interval - most free services offer 1 hour monitoring interval but you can also get a monitoring interval of 15 mins
• False Alarm protection - this means that the site would be tested from various locations to be doubly sure that the website is really down
• Contacts to be Alerted - sometimes it may be necessary to recieve the alert at a different email than the one with which the login was created or to receive the alert at multiple email ids
• Performance Reports - Daily, weekly, monthly reports provide useful stats

Generally free services do not provide the whole set of facilities. I am using both InternetSeer and SiteUptime to monitor my website :D. Two other services which could probably be conisdered are (I have not tested them) Killer Website Monitor and MrAlert Free Website Monitoring Service.