I tried this out (July 2022) and unfortunately, pow.cx no longer exists.
If you would like to use a "real" domain name in order to visit your Rails sites on your development environment, you can just use something like http://lvh.me:3000 - lvh.me directs to the IP address 127.0.0.1 (your localhost). The advantages of this are
it accepts subdomains (so http://customer1.lvh.me:3000 works just fine),
it doesn't require any configuration or software installation, and