I’m not a fan of 37signals, but I must admit their DNS service xip.io is handy. I’m setting up some web servers right now, and I needed a domain to test my configuration. The whole DNS dance is a bit time-consuming: add record to zone file, & wait for my DNS to pick it up. With xip.io there’s no need to wait: prepend your host’s IP address before .xip.io and the domain will resolve to your own IP.

For example 127.0.0.1.xip.io will resolve to 127.0.0.1.

They are other services like this like ipq.co or localtest.me, but as far as I know they don’t work out of the box: you have to register your subdomain first or can only use it with localhost.