Wheels were used in a variety of ways. For example, a person could be somehow attached to the outer rim of a wheel and then either rolled over sharp spikes or off a hill.

Crucifixion

Most people immediately think of Jesus. Early crucifixions were carried out on trees, later to be done on crosses in the AD era.

Drowning

Used in a variety of ways. Popular at sea and witch trials. Probably the most well-known variation is the witch's test. If the water rejects the woman and she floats, then she is guilty and executed by anth method. If the water accepts her and she drowns, then she was innocent.

Blown from a Cannon

The person is positioned across the mouth of a can, then a shot is fired, blasting through the person's body.

Pendulum

Just like in the Edgar Allen Poe story. A person lies on his or her back and a razor edged pendulum swings above them, slowly descending. There is question if this actually existed, or was just a creative work of the imagination.

Poison

Interestingly, this has not been a common method of execution. Socrates, who was forced to drink hemlock, is probably the most well-known victim of poison.

Pressing

The person lies under some sort of slab-like surface and then heavy objects such as stones are placed on top until the person is slowly crushed to death. This process could take hours, or weeks to die from.

The Rack

Catch-all term for a number of mechanical devices and practices which cause torture and death by stretching the person.

Sawed in Half

Not just used in magic shows

Death by Insects, Etc,

There are many variations on the scenario in which the person is staked to the ground, smeared with something sweet like honey, and left out to eventually be eaten/stung to death/devoured by insects. This was a favored tool of the Muslims, and some Native America Tribes.

Disembowelment

The torso is slit open and the entrails are removed.

Shot by Arrows

Practiced by the Vikings. They would aim at non-vital parts of the body in order to prolong agony as long as possible.

Spanish Donkey/ Wooden Horse

The person sat astride a V-shaped structure as if on a horse or donkey. Weights are increasingly attached to the person's feet pulled increasingly downward, until the person is split in two. Also used modernly as a tool of sexual torture in the SM movements.

Starvation

The person is put away in a cell or cage and not fed anything. For example, in medieval France there were oubliettes (oublier is France for forget). Currently, the CIA still uses this method while making them stand at attention during the duration of it all.

Stoning

Rocks are thrown at a person until he or she dies. Often members of the community will assist in the stoning. Still in practice in some Islamic and African  countries (or at least until very recently). Some great fictionalized depictions of stonings include Zorba the Greek and The Lottery by Shirley Jackson.

Garrote

Oftentimes a hybrid of the rack and hanging. Something is tightened around the person's neck until death results.

Dragging

The person is tied to a horse and dragged until death.

Throat Slitting

Not used very often because of it's lack of drama.

Thrown from a Great Height

The person is thrown off a mountain or a high wall.

comment: "Being thrown from a great hight" nowadays includes being thrown from a helicopter (unofficial method by certain military organizations)

Tied in a Sack with Animals

A favorite practice in Ancient Rome, a all time fave of the Spanish Inquisition. 

The Cauldron

 A iron container, mouth down, is placed on the victim's stomach. The container is lifted a bit and some mice or rats slipped inside. Then the container is heated up from the outside with a flame. As it begins to get hot, the mice want to escape, but there is no way out except through the mouth of the cauldron. So they begin to gnaw through the person's stomach and do so until the person dies.

Torn Apart Between Two Trees

As in the Robert Frost poem "Birches", two trees would be pulled down so they would come together from opposite directions. One arm and leg would be tied to one tree, the other arm and leg to the other tree. Then, let go of the trees.

Torn Apart by Horses

A person is attached to four horses, one to each limb of the body. The horses are then giddied on to gallop in oppostie directions, in the aim that the person will become dismembered. Not a very effective method unless certain tendons in the limbs are pre-cut.

Water Death

Person is forced to drink water until death. Traditionally a stiff tube (made of intestine) was forced down the victims throat and they had a non-stop flow of water entering them, or a slow trickle depending on how long they were to suffer.

Water Torture

Used in U.S. prisons in the nineteenth century.  Water was poured on top of the prisoner's head and a large bucket of water was also placed under their chin to simulate the feeling of drowning.

The Gag

This device was placed in prisoners' mouths and kept in place by locking the chain around their necks. Used in U.S. prisons in the nineteenth century. People starved to death via the gag.

The Cage

Giant oddly-shaped metal cages were placed on prisoners' heads and worn throughout the waking hours. Used in nineteenth century American prisons and asylums.

The Cat/Flogger

Prisoners were whipped with leather straps and cat-o'-nine tails until their flesh was raw and bleeding. Used in nineteenth century American prisons.