So I've just done a serious comparison of the Overstock.com "deluxe memory foam mattress topper" (it comes with a little card that says "sleep comfort systems by Leggett and Platt") and the "memory foam mattress topper" from Target, made by Sleep Innovations.
Both were the two inch thick models, for a queen-sized bed.
The one from Target ($70) was several inches too small in both dimensions (you can tell by looking at the box; a queen-sized bed is 60" by 80", this thing says on the box 55" by 77"). From Overstock.com I got a nominal California King topper ($80), which is 70"x80" and can be cut down to the right size.
The Overstock topper is heavier foam, 4lb density; based on box weight, the Target topper is probably 2.5lb or so. However, the Target topper still manages to be more comfortable and more supportive. The Overstock topper seems to be more "regular foam" than "memory foam" and just sort of squishes down. I can sometimes even feel the springs of my bed through it.
Based on comfort, Target wins hands-down. Based on fitting the bed, buying from Overstock and trimming is the way to go.
As an aside, for those who don't know, the reviews on Overstock.com are totally bogus. They mostly only post the very positive reviews, with just one or two less-favorable ones thrown in to make it look legitimate.
I hope this is helpful.