I'm looking for a good Uptime Web Host monitoring service that sends a request as a user to our websites and have the ability to text/email in the case of outage or website down.

I have been evaluating: http://www.host-tracker.com but will not go with their service as it has been reporting 100% uptime. That's a great stat, but in the last 2 days, we have been making significant improvements to our data center and colocation... and I know when the websites were down, but this service reports 100% uptime. This leads me to believe the service is not good.

So does anyone know of a good reliable service?