Based on a great thread from Friday, we've gone through and put together this list. Thanks to everyone who contributed and feel free to add to this post:
Overstock Premium Memory Foam 4-inch Mattress Topper ($200)
Overstock Memory Foam 4-inch Mattress Topper ($170)
Talalay Latex Toppers in 2" or 3" (from $222) This is natural latex, not visco-elastic.
Kohl's Sonoma 4-lb. Memory Foam Bed Topper (from $180)
BB&B Therapedic 3" Memory Touch Mattress Topper ($200)
3-inch Isoform Memory Foam Mattress Pad ($152)
(Note: "You should learn from my mistake: the sizes on O'stock are cut WAY too small for an average mattress. I got a King for my Queen and it was still several inches too short. Highly recommend ordering the California King and cutting it down to size with an Xacto." Sara B.)