Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.

WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.

Trello Calendar Connector by Cronofy - See and Organize your Trello deadlines inside your calendar

Oh Dear! - Website monitoring, without the hassle

Agile Guy for Trello - The Swiss army knife for agile teams!

updown.io - updown.io is a HTTP(S)/TCP/ICMP monitoring service which checks periodically the URL you want and reports back any anomaly.