I had one of these on my bed for 5 or so years... really awesome! I sleep great with it. I need to get another now that I've got a bigger bed.
try to get one that is at least \4 lb memory foam, the denser foams are a lot more supportive. and comfortable. a lot of the cheaper memory foam pads you see in stores are 2lb
Serta Ultimate 4-inch Memory Foam Mattress Topper | Overstock.com