Common pillow stuffings include feathers, down or a combination of feathers and down. Other fillings are polyester, polyester pellets, microbeads, foam and tiny polystyrene beads. Pillows can also be stuffed with cotton, wool, horsehair, shredded rubber and seeds, such as buckwheat, flaxseed and millet. Lavender is also used as pillow stuffing.
When polyester is siliconized it feels very much like down. It's also hypoallergenic, washable and readily available. Polyester pellets are often used to stuff toys but can be used in pillows of many sizes. Microbeads are even smaller than pellets, and are used for therapy pillows because they are almost frictionless. Beanbag filler is made of light, fluffy polystyrene beads. Foam is famous for adjusting to the shape of the sleeper's body.
Like polyester, cotton is also hypoallergenic, but over time it does tend to mat down. Pillows stuffed with nothing but down are the most luxurious, but they are also expensive. Down pillows also can't be washed safely, so the pillow must have an extra covering. Down is often mixed with feathers, which are flatter and stiffer, adding needed support.
If the pillow has a good liner, wool feels as soft as cotton. Wool is renown for its durability. Horsehair naturally wicks away sweat, and shredded rubber feels very much like memory foam. Seeds and herbs are often used for smaller and therapeutic pillows.