Pingdom is a service that monitors websites and servers from the Internet and alerts you if there is a problem. It acts like a watchdog, making sure that your website is up and running 24/7. You handle all your settings from an online control panel. No need to install anything. You can get error alerts via both email and SMS.
Now, you can use Pingdom for free. It is not a free trial, but a completely free account that you can use for as long as you like. Pingdom Free has all the features of the paid account types. The only limitation is that you can only monitor one website or server, but that should be enough for a lot of bloggers and hobbyist webmasters.