Nothing. My 1st choice is kneel (Really sitting), second prone, 3rd standing (if a tree is next to me I'll lean on it as it is the hardest position of the three).
It really isn't that hard to learn to shoot this way. I'm really somewhat shocked at the dependency that there seems be on the other choices now days.
There have been "programs" (Like ISSF shooting events) and competitive events around for years that teach people to shoot the 3 positions proficiently. The boy scouts was associated with them when I was going up. They start with airguns at 25 yards (10m now) with 1/4" targets, then move out from there to 300 meters.
If you start kids out early and teach them these methods, they won't need to extra tools. (Besides the fact the events are fun as can be for all ages) Having to get your crutch in the right position can make you lose a shot, or limit how many targets you could have shot at.
All that said if using something to rest your rifle/pistol makes you get a good kill shot, why not use it. Its all good.