I've tried several of the off-brand pillows (from target/bed bath and beyond/costco). and they generally were pretty bad. no matter how cheap/pricey. either too firm/too hard, and commonly very poor at releasing heat.
My friend had a tempur-pedic mattress topper lying around for our first 2 years of college... I've used it several times and it was very comfortable. A few months ago, my family bought a foam mattress topper from another brand at costco. It currently sits on my bed at home. As far as I can tell this one is comparable to the tempur-pedic.
Unlike the off-brand foam pillows which when compared to tempur-pedic foam which was nowhere near in quality, this other brand I found at costco was quite close in quality. It's comfortable, and I haven't had issues with the heat building up and not getting released. Although, I have not been able to use the bed this summer, since one of my siblings has taken to sleeping on my bed while I was gone at college. (He can sleep through anything, so I can't really attest to if the foam has trouble releasing heat in summer conditions) I don't think we have the box anymore that it came with... to help others identify it... I do believe it's advertised as "comes with a pillowtop mattress cover".
--edit
I live in southern california, both my school and home-home locations are in SoCal. to help those gauge my surrounding temperature if you so wish to know.
and whenever i refer to "foam" in my post, I meant the "memory-foam" not sure if there's a difference.