Alternative to Pingdom website monitoring service
Check into UptimeRobot http://www.uptimerobot.com It's free, offers HTTP and ping monitoring every 5 minutes with email, text and RSS notifications. To my soon to be former webhost's chagrin, it's very reliable with uptime monitoring logs and past event tracking.
https://uptimerobot.com/ offers a free plan with 5 minute interval checking, with one-minute interval checks on their paid plans.
If you're really concerned with uptime you might also like to consider DNS hosting with a DNS failover service, which monitors your site every 2 to 4 minutes and updates your DNS to point to a backup server should it find that your primary server is down. Of course, this means you need to pay for a backup server, and that you need to mirror the content between the two servers.
mon.itor.us and SiteUptime have free/ad-supported plans. Check are only at 30-minute intervals however.
They both have plan upgrades which measure at 5 minute intervals and are cheaper than Pingdom, so you might find a reasonable cost/benefit trade-off.