Email services in general. My worst nightmare is my email provider (which isn't Google) going dark and losing access to everything.

Realizing this is why I bought my own domain name and pointed the mx records at Gmail. This way I can change it to different mails servers if needed, even self hosted. One useful thing you can do is configure Gmail to forward mail to unknown address to a known one. So I can create addresses like [email protected] or [email protected], etc