The Knife

Dexter uses variety of knives through whole series so be more specific in terms of episode :) I think, that SOG Pentagon was most recognizable - he used it on Lila at the end of season two. Pentagons come in two designs - older (the one from the movie) is hard to get, what's worse most internet shops display picture of old Pentagon, but they will probably ship you a new one, so if you are like buying one - ask first! I have both of them - beautiful knives, but those are "born killers", not utility knives, so I rather discourage you from carrying any of them around town. Besides each of them (including mini versions) is double edged, so check your local law is it legal to posses such a knife.

picture: http://www.specshop.pl/images/SOG-Pentaago­n-1­.jp­g