If you have your website, e-mail system, etc. running on your own VPS or dedicated server it’s good to know when that server is unavailable so you can contact your server provider asap.
The best way to do this is use an external monitoring company. I’ve been using Hyperspin for a couple of months now and it’s been working great.
You can pick what services/ports on your server to monitor (SMTP, HTTP, HTTPS, etc.) and you can chose how to be alerted when they are not available. Obviously you need to make sure that you’re alerted via an out of band method (in other words don’t rely on an e-mail to an e-mail account on your VPS to know that your VPS is down!)
The Hyperspin website isn’t very pretty but overall their system is reliable and easy to use.