Memory foam and latex foams ...
The two biggest technologies are memory foam, which is now around 30-40% of beds sold, and then the latex foams, which are newer.
Memory foam is nice, conforms to your body, great for cooler climates. Depending on your weight, some brands -- even the very "household name," costly brands -- will sag and wear poorly over time. A few have even firmer and thicker for less, direct to your house. If you're looking for memory foam.
The other segment, which is also growing, is latex foam. I have a latex foam. I live in a warmer climate and I'm 220lbs. these days (about 50lbs. heavier than my most athletic, 3-sport form in high school). It breathes better, cooler around my body and will last longer. Serta has been pushing theirs pretty hard in the stores, and its cheaper than the "household name" memory foam too.