Mailing address:Monkey See Monkey Do SIA
healthchecks.io is an open source project. The code is on GitHub, it has BSD license, and is accepting contributions.
The service is currently run on Google Cloud Platform. The app servers are load balanced, but the Postgres database currently is a single point of failure. The database does have daily backups. The ops team consists of a single person, so multi-hour or even multi-day outages are possible!
While we can only guarantee a best effort availability, in practice the availability has exceeded 99.9% since the service publicly launched in July 2015. That is, until 20 August 2016, when the service experienced a 24 hour outage.
There's also a number of alternative cron monitoring services, with different feature sets, different reliability guarantees and pricing options. If healthchecks.io doesn't seem like a good fit for you, why not check them out: Cronitor.io, Dead Man's Snitch, Coal Mine, PushMon, steward.io, CronAlarm, Watchdog Timer.