An uptime monitor is a service that acts like a visitor to your site to check if your site is up. You can schedule the service to visit your site as often as you like. I find five or ten minute intervals to be comfortable without using a lot of bandwidth.
They hit your site and notify you if it’s down. Depending on the service you can get an email or SMS text message when your site goes down. They use different servers around the world and can also tell you how fast your site is delivered.
It’s reassuring to know if your site down and start doing something about it. Don’t count on your hosting company. I recently had an issue with a site being unavailable, but because it was a network issue and not a server issue, my hosting company wasn’t aware of it. My monitoring system notified me and I was able to create a support ticket immediately.
I highly recommend pingdom.com. They offer a free version that’s everything you need.