Safety Check is a simple plugin used to notify when the last player has logged out of a world. It creates a file based on the configuration containing a timestamp. I use this as a way of determining good times for backups. On larger servers, this will probably be useless as there are always players on. Because I run a personal server in a non-environmentally controlled area, it is subject to natural phenomena (such as my two year old pushing the power button, which the server doesn't like.) Because of this, I want to make a backup every time the world is empty, but not on a regular schedule, as it doesn't make sense to backup a world that hasn't changed. So it would seem to me that a good time would be when the last person had logged out.
The only configuration is the outputFileStr which is a string format that will be used when outputting the timestamp file. By default the outputFileStr is all_logout-WORLD. The plugin will replace WORLD with the name of the world where all players have logged out.