M.C.
Simmons Lifetime. It was around $120 at Babies R Us. Similar to this one: http://www.toysrus.com/product/index.jsp?productId=345226...
I chose it because it was firm but not a rock. One side faces up when under 35lbs and then you flip if over and its a slight bit softer. My daughter is almost 5 and we just stopped using it last week when we changed her toddler bed to a full size bed.
I did notice that some mattresses had a built in vibrating/soothing setting. I don't like those. I think that they 1. make it so that the babies HAVE to feel movement to sleep which I don't like, and 2. I'm a worry-freak about SIDS and such.