This morning one of my co-workers came into my office and told me how some asshat stole his truck's tailgate in the middle of the night. This reminded me of a couple of security products that have been mentioned on this site in the past, so I figured I'd start up a thread regarding security measures for our trucks.
Two products I highly recommend - have used them both on my trucks in the past - both from the same company.
1) Gate Keeper - this is a metal bracket that is bolted onto your tailgate that helps cover up the notched side of the tailgate that allows you to remove it from your vehicle. Installation takes less than 5 minutes. This is handy because it makes it impossible to simply walk up and remove your tailgate - requires an allen wrench to remove the bolt/bracket, which most thieves probably aren't going to bother having on hand when they're trying to sneak up and steal your tailgate in the middle of the night. If you do need to remove your tailgate for towing/moving/whatever, you can remove/re-install the bracket at will.
2) Jimmi Jammers - this is a metal plate that is installed inside of your vehicles doors, between the plastic door handle and the lock mechanism/rods. Some thieves will attempt to access your vehicle by punching through the plastic handle or the thin metal around your handle with a large screwdriver/punch, then use a pair of pliers to lift the latch rod and open the door. These plates block access to the rod/lock mechanism. Obviously this won't stop the thief from trying another way to get into your vehicle (eg: smashing a window), but IMHO anything that helps deter them or delay them from getting into the vehicle can help - might make them give up and move on to someone else's vehicle.
Both products are available from http://www.jimmijammer.com