Since 2.1, huntersmark is stackable, but u cannot see the stacks on the buff icon (thx blizz btw).
Is there an addon out there that shows me the stacks?
Hunters Mark is not stackable in the sense that you cast it multiple times, the change is that everytime you hit and do damage it increases your attack power by a %. There have been some suggestions for an addon to show you that increase. I have a hunter and have looked at what might be required but with work being ramped up due to impending release and working on a different addon I can't work on creating something for that now.