One of my all-time "tactical" folder favorites is my Spyderco Yojimbo.
I tried not to like it, I really did, but it grew on me.
It's shorter than legally permissible, but still deadly and quick, and yes, its great for opening boxes too.

Recently, Ive been using it for
dangerous things like cutting cardboard and plastic sheeting for a jaycee haunted house.
The only problem I see is it is almost impossible to close it one-handed due to the location of the lock.
The knife is no longer a production item, but I encourage people to seek them out.