Memory foam mattress appeared relatively soon. It is also known as "Novaform mattress", "Visco Elastic" and "Contour mattress". You maybe heared about it as of "Nasa mattress". You can ask "why Nasa?". Because memory foam, material which is used in mattress manufacturing, was originally designed by Nasa.

So whats so distinctive with this type of mattress ?

Under the influence of body heat and weight, memory foam changes its shape to fit the shape of a warm body. It is also pressure-sensitive and responsive.

This time I start my review from "cons", not "pros", and here is the reason.

Cons of memory foam mattress:

It feels like sleeping in a piece of plasticine. Most of the people don't like it, but some adore it for this reason. Sometimes because of "slow rebound" you can't comfortably turn - it remembers your previous position.
Memory foam has distinct chemical odour. Some people complained that they have allergy symptoms using this mattress.
Good memory foam mattres is expensive.

Pros:

Useful in hospitals for medical-related uses.
Useful for old-people or people with spinal/neck disfunctions.

Selection tips:

Get one with good surface and without chemical odour.
Choose the one with right support level, because they differ.
Certainly get one with money back trial period for at least a few weeks. Many customers complained that memory foam looses its initial flat shape in 1-2 years.
Get a good warranty.
Take it from one of the world known Brands.

Conclusion
Production of high quality memory foam is relatively expensive process, so stay away from cheap / non-brand memory foam mattresses.
Keep reading my articles and find out true about other types of mattresses.