Clamshell Hand Post Hole Diggers
The most common hand post hole diggers are the clamshell type. You drive the two blades into the ground, spread the handles to close the blades, withdraw the tool and bring the handles together to dump the soil. This type of tool can dig a fairly small diameter hole (4-6 inches) to a depth of 1-2 feet. If you try to go deeper, the small diameter of the hole prevents you from spreading the handles, thus you can't pick up any more soil. This type of post hole digger can be used next to walls, along fences, etc. more easily than any of the other types. This type of digger is the least expensive to purchase.
Auger Hand Post Hole Diggers
A better alternative in many cases is a hand-operated post auger. This tool consists of a short tube with an auger section on the bottom and a T-handle on top. It will typically dig a 6- to 8-inch diameter hole. You basically just turn the T-handle to screw the auger into the ground. When the short tube is full of soil, you pull the tool out of the hole and dump the soil. Theoretically, there is no limit to the hole depth with this tool since you can add additional handle sections (simply lengths of steel pipe) as needed for deeper holes. If the subsoil is too hard for the auger to work properly but the hole is too deep to use a clamshell digger effectively, it is sometimes helpful to use a clamshell digger to chip the hard subsoil loose and then use the auger to pick it up and clean out the hole. With either hand-held post hole digger, it is helpful to mark the depth on the handle so you can tell your depth without measuring the hole.
Handheld Power Augers
If you have several holes to dig, you might consider renting, or even buying, a handheld power auger. Some handheld auger are designed for one-person operation, but many require two people. These tools may not involve quite as much work as hand-operated tools, but they do still require a lot of strength and effort, especially in hard clay. Safety of handheld power augers is a concern. It takes a lot of torque to spin an auger through hard soil. If the auger hits a root, rock, pipe, etc. and jams, that same torque can spin you around the now-stationary auger -- or break your arm. You also need to avoid loose clothing that might get entangled in the auger.
Wheeled Power Augers
A somewhat safer and easier to handle design adds a frame with two wheels to one side of the auger and an extended handle to the other side. These machines are available for rental. They have about the same capacity as the handheld power augers but can be operated by one person and require somewhat less effort. They are still not easy to use in hard ground.
Tractor Post Hole Augers
The solution that requires the least effort on the part of the operator is a tractor-mounted post hole auger. These machines mount to a tractor 3-point hitch and are driven by the tractor PTO. Even small, compact utility tractors can handle a post hole auger. These machines are safer -- if the only operator is on the tractor and doesn't get off while the PTO is operating. No one should be within 25 feet of the auger when the machine is operating. Tractor post hole augers can be purchased for a few hundred dollars or rented.