Monitoring website up-time

Most web-hosting firms specify the level of “up-time” you can expect to get on your website. Typically this is given as a percentage (e.g. 99.99%, 99.999%, etc…), but how do you know if they’re really telling the truth? After all, you can’t measure this yourself.

I currently use Pingdom to measure up time for my sites. Pingdom constantly monitors websites and produces a monthly report stating the uptime. It also emails me to tell me each time my site does not respond for a period of time (i.e. it is down), and when it comes back up again. The basic package is totally free and perfectly adequate if all you want is to receive up-time statistics and site-down alerts.