The JuiceTank is a combination iPhone case and charger with a built-in electrical plug that eliminates the need to carry a charging cable.
Its two prongs lay flat on the back of the case, minimizing any extra bulk, and then pop out when needed using a spring-loaded mechanism.
Granted, a cable can be nice when you want to use your phone while you're charging it, but cables are easy to lose or forget. It's a lot easier to remember to bring your charger with you when it's attached to your phone.
The JuiceTank is currently seeking funding on Kickstarter, and has raised $25,000 toward its $125,000 goal with 54 days to go. If it gets funded, the case will be released this summer and retail for around $70 (although backers can get one at a discount).