The uNu DX Protective Battery Case for iPhone 5 doubles your talk time, letting most iPhones last for two days on one charge.
While that's still far from what feature phones can manage, it's a significant improvement and would come in handy in many situations. The great thing about this case is how little bulk it adds. It's 15mm thick and actually looks quite good.
Another case discussed in the video below is an LED case that's still a prototype. It increases your battery life, protects your phone, and adds an LED panel to the back of it. The cool thing about the LED panel is that you can connect to it with Bluetooth and push custom notifications to it.
You can already use the the built-in iPhone flash for alerts, but with this case you would get different notifications from each app.
While I don't care about getting better notifications on the back of my phone, I'm sure some people will see the benefits of having the LED panel.
There's no doubt that uNu is doing interesting things with cases. Learn more about what the company's up to here.