Favorite addon to my 08 Ford F250 is the Bakflip bed cover.
http://www.bakflip.com/ It keeps the Superglide hitch covered when not in use and I just flip it forward to hookup and go. It has worked flawlessly and keeps the bed dry.
My next addon will be a gravity feed RDS Auxiliary Fuel Tank (32-Gallon Capacity, Model# 71804). It will fit forward of the 5er hitch and is only 9" high so it will fit under the bakflip cover.
The silliest addon has been this removable "Dog Shelf". We travel with two small dogs that like to look out the window. They were scratching up my leather armrest, so I built this carpeted dog shelf for them to ride on. They love being able to lay down and see out the window. Silly but effective.
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