Alertra is a website monitoring service that monitors and detects outages on the fly for VickeryHill's support staff.
We have been using it for over 4 years and found that, at the lowest price point of $12/yr, it is one heck of an insurance policy for websites hosted all over the Internet. We use Alertra to monitor:
When an alarm goes off, we (and our clients) get alerted via:
When alerted, a VickeryHill staff member disables the monitoring (so the phone stops ringing) and then begins working to to determine what the problem is. Sometimes it involves manually checking the website, testing the FTP and file structure, verifying domains and the registrar and then usually contacting the hosting provider to solve the issue. It really is a nice, inexpensive solution.
Let us know if you are interested in having us monitor your site.