There is a nice and free website monitoring service available – Pingdom GIGRIB.
This website shows the uptime information for the thousands of websites that have been added by our users. The information is public and searchable.
All you need to do is :
a] Create a Pingdom GIGRIB user.
b] Download and install the GIGRIB client
c] Add the websites you want checked in the Pingdom GIGRIB control panel.
However your web site uptime information remains public. You can try the commercial service has a separate network of monitoring servers, more features, and the information is not public.
=> Pingdom GIGRIB website.
It is a great tool. but there is another one. try Dotcom-monitor