I love my iPhone 4 to death. I really do. Even if it sucks juice faster than a 1st grader at snack time and I often find myself on low power with a long train ride ahead of me.
I originally made my Altoids USB Charger to use with my iPhone, only to find that Apple being Apple doesn't let it's products play nice with generic USB chargers. I then set out on a long journey to find a cheap charger that would work with an iPhone 4. After sacrificing many a cheap Chinese charger to my garbage can, I eventually found a great little circuit with the added bonus of having a retractable cable.
It's a cheap and easy project to put together and a great gift to give.
*** Update: I've since retired this kit. It's not held up over time very well. I've done an updated version called Solar USB Kit 2.0 and a more rugged version called Lithium Heavy Duty 2.0. If you're looking for something pre made, especially for camping or emergencies, you should try out one of our Folding USB Solar Cells. They're inexpensive and much much more powerful than what you'll find here.
Time: 30-60 minutes
Cost: Under $20