Spigen Thin-Fit ($11)
Spigen makes cases in just about every size, shape, material, and more for buyers. Basically, they'll have something you'll want.
The Spigen Thin-Fit is a new case style designed to cater to those who want some protection but hate using a case. If you don't want to add any bulk to your phone, this is a good choice. They're using a very thin, slightly flexible, but strong polycarbonate plastic.
This is perfect for the minimalist, but keep in mind that others will offer more protection. The buttons are just cutouts, not covered, so the edges aren't all that safe. It's a thin case for someone that just wants a tiny peace of mind.