Well, your comment about the 'root cause analysis' suggests you need something more substantial than this, but I've been very pleasantly surprised with www.uptimerobot.com.

I have both Pingdom and Uptime Robot doing basic monitoring of a single web endpoint and Uptime Robot consistently alerts me to downtime by email faster than Pingdom.

Because Uptime Robot is free and doesn't have (lacks?) a business model I didn't take them that seriously. Their performance suggests otherwise.