What are some alternatives?
When comparing Pingdom and AlertBot, you can also consider the following products
UptimeRobot - Free Website Uptime Monitoring
StatusCake - StatusCake | Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
Apex Ping - Uptime & performance monitoring for websites and APIs.
RapidSpike - RapidSpike is an uptime and performance monitoring service for websites and web applications.
updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.