Saints Row the third is great fun and the closest you'll get to Postal (not very close mind you, but about the best I can think of for now). One of the level designers that worked on Postal 2 is the lead (I think) Level designer now for the guys that made Saints Row the Third too, so it's kinda connected that way. Thankfully, there is not and hopefully never will be anything else that resembles the disaster that is Postal 3
Yeah SR3 is fantastic, it's an SDK away from being my favorite of all time.
There is acualy one game , that for me is a little similar to postal games.its called harvester and its quite old game (1993 or someting like that)it is quest, but u can run aruond killing everyone(for quest its strange , but it works)this game was banned practicaly in every country, but can download it somewhere ,its not a problem.you will nedd doss box to run it,but grafics is photorealisticand pretty good.be carefull playng this game <it is more violent and crazy than postal (imo)