It's really not strictly a case, but I use a Mophie Juice Pack Air battery extender, which is basically acts like a hard plastic case while giving me extra battery life. It adds a little weight and thickness to the phone, but it's not that bad and I can get through a long day of using the phone a lot without making a big dent in the iphone's own internal battery.
It doesn't have a belt attachment or anything so I just picked up a generic horizontal leather belt holster thing I found at Target that I can slide the phone into when I need to.