Chaos reigns on web-based services if you let domain names, SSL certificates or other key infrastructure expire. certalert.me is a service that warns you if a company-owned domain or SSL certificate is about to reach its due date.
Simply enter the relevant domain details and a contact email and the service will issue you with a reminder. You should be tracking such critical infrastructure in other places, but it certainly doesn't hurt to get an additional nudge. Note it won't work with everything (we had no luck with .com.au domains, for instance).