Pingdom is basically a service that monitors websites uptime and if there is a problem it alerts you via email or SMS. Pingdom allows you to feel secure in your knowledge of how your site is doing from all over the world.Their service can check if your website is up every minute ,if it is down at one location,they make tests from several locations around the world to make sure it is really down.In other words we can say Pingdom as a Watchdog of your website.
Normally a Basic pingdom account will cost around $10/month.Now, you can get full featured Pingdom account for free.You must note one thing,this is not a 30 days trail account,this is different. In this free account you will have all features of the paid account,like 20 SMS alerts,the only limitation is that it you can monitor only one website.i.e one website for one(email) account.
I think you can grab above tip,if you want to monitor 2 or more sites.