This could help, but you need your own server to do it. If you have a raspberry pi with a webserver installed then its perfect for it. http://sourceforge.net/projects/phpservermon/
Just bear in mind there is a lot more to System Monitoring then simple ping monitoring. Are your web sites serving up pages? What is your CPU Load? Your Disk IO percentage? Your swap usage? Many things need to be monitored to provide a better view of a server's availability and performance.
Not sure if you want a full system monitoring or a ping service.
Monitor.us has a free service with agent for server monitoring (limited).
If you want a free ping service you can try http://uptimerobot.com/.
Checkout Pingdom.com or UptimeRobot. (Please note: I have seen many medium and large businesses use Pingdom).
We currently use UptimeRobot as Pingdom was notifying us of downtime in our servers in which never occurred and our servers were still accessible. Both services will send you email/text messages of downtime.
We use StatusCake.
Their API allows us to automatically register and monitor everysite as it's added to an account in the control panel.
We moved to SC after we had continued issues with pingdom a while back.
We figured if the monitoring service was down more than we were, what was the point.
Having said that we were happy with pingdom up until that point.
But false alerts incorrectly telling customers their site are down is not funny.