Instructables has quite a few guides on how to make Solar USB Chargers, including the very well done guide on how to combine a Lady Ada Minty Boost circuit with a solar + lithium ion battery. Great, but a bit expensive to make and not a very simple project for the weekend DIY person.
Well luckily for us I know how to make one for under $20 that is better in nearly every way and also completely fits into an Altoids Tin. Covert style.
(If you want a more powerful USB Charger, a Heavy Duty one, I have an instructable for that as well.)
Step 1: What You Need
USB Charging Circuit
Solar Panel 4V or greater
AA Battery Holder
AA Rechargeable Batteries
Altoid Tin (or whatever)
Melt Glue Gun and Glue
Cost is less than $30. I can make one for under $10 when I buy parts in bulk. I have a kit available at my website BrownDogGadgets.com which has everything you need to make this project. Also, if you're lazy, I do sell made versions in a variety of tin styles.