SSL Alerts - Free service to send SSL certificate expiration alerts
CloudFlare - Cloudflare is a global network designed to make everything you connect to the Internet secure, private, fast, and reliable.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Google Project Shield - Google's DDoS protection service
haveibeenexpired - No more expired SSL certificates
SSL Remind - Never forget to renew your SSL certificates for free!