I've never used DNS in my local network (because it's additional burden to support, so I tried to avoid it), but couple of month ago when I needed several internal web-sites on standard http port, I've just came up with "localdomain."
Yep, it's non-standard too, but probability of it's usage of gTLD is lowest among all other variants because of it's usage in Unix world and how non-pretty it is :)