Visco-elastic foam (aka memory foam, or Swedish foam) is rather odd stuff. When you poke it, it takes a while to gloop back into its original shape. This isn’t the neat bit though. The neat bit is that it becomes softer at higher temperatures, so it can react to human body heat. The result is to spread pressure very evenly over the bits of you that touch it.
It’s currently used mainly as pillows and mattresses, but could probably be used for anything that exerts pressure on the human body. I recently bought a pillow and mattress topping made of the stuff… very nice I’m planning on using it for a seat cover and to pad my backpack straps as well now.
In terms of working with it, it seems to be just like ordinary foam except slightly heavier. I cut a slice off my mattress overlay with scissors: no problems, nice clean cut. At a place like Target or equivalent a memory foam pillow goes for $35, a (fairly thin) mattress overlay for $90. A company called “Tempur” has been marketing the stuff fairly heavily.