[The following tool is used as an entry point for monitoring certificate expiration, which is useful for checking your servers are in good shape. ]
People have realized that certificate transparency - logs of all issued certificates - can be used to find hidden domains. So far, all sources required some level of programming.
This time, you can just keep typing domain names and see what you can find.
You can also have a look at this 50s video before you try for yourself.