Ogma says: I've heard they are a great alternative to expensive mattresses, revive mattresses, are good for aches and don't have the dreaded bounce inhibitor effect of full on foam mattresses.
We have yet to use one, but have considered it. Here is a starter list of pads from Overstock.com. What's your opinion? (Thanks, Ogma!)
Premium Memory Foam 4-inch Mattress Topper (All Sizes)
Memory Foam 4-inch Mattress Topper