Most of them are patched-up versions of the original Kronzky single player file (which only worked up to DayZ version 1.7.2.5). There are videos out there on YouTube that explain how to get single player working with DayZ. Sadly, for various reasons Kronzky and Rocket (the DayZ developer) fell out and they both decided that the file should be discontinued. It was pretty good and added a whole new dimension to playing DayZ.

Once upon a time there was a single player mod file for DayZ written by a chap named Kronzky.