Buy (or dig out of your closet) an old Wifi router and install Tomato on it. Its web interface lets you edit its hosts file which is then active for all the devices connected to it.
Except many of us don't have Tomato/DD-WRT capable devices, not to mention I can't be bothered setting up an additional router to my current two, flashing and securing it and then switching my devices over to it just to test something when this solution does essentially the same thing.
Because it's a PITA to show other people how to make entries to their hosts file if you want for someone else to look at your project. I'm not telling you anything you don't already know, but this is why I would use this service over just making entries to my own hosts file.
