Adafruit Solar Powered SKULL Blinky LED Pendant Kit from Lumen Electronic
Scary. Solar. Skull-y. The Solar Powered SKULL Blinky LED Pendant by Lumen Electronic Jewelry is an awesome new kit where, with a little soldering, you can make a battery-less blinking skull. That's right, there's no coin battery! Instead, a 1 Farad super capacitor is used to store energy. It takes around an hour to make and, when you finish, you can charge it either with a USB or with the power of the sun.
It takes less than 2 minutes or so to fully charge through the USB and 15 minutes from direct sunlight. In darkness, it'll last for almost 90 minutes - making it the perfect goth accessory for outdoor Disneyland adventures or indoor Bauhaus concerts (you can charge it during the opening act).
There are detailed assembly instructions available here! And the pendant also comes with a schematic and parts list. The photos above show one that was dipped in clear-coat epoxy to protect it after assembly.