Be aware that many of the places that offer these ping services for free are only monitoring things every hour - so if a site was offline for 3 minutes, you may never know about it.
There's a few out there that do more frequent monitoring, and then also monitoring from multiple locations.
I like
www.freesitestatus.com as they monitor from several locations and can monitor different services every minute. Runs at $12/month per URL monitored.
www.host-tracker.com is another popular place, to get into the 1 minute interval tracking range you're looking at about $30/month for up to 20 urls.
Pingdom has some great monitoring too.