Guyot Designs LightCap 200
Here's what other sites are saying:
Just replace the cap on any standard (2" wide mouth) water bottle with this light-weight, ingenious cap and you'll have light all night long. The polycarbonate LightCap200 has four super-bright white LEDs for lots of light and the tiny built-in solar panel charges the replaceable AAA NiMH battery to full capacity with just eight hours of sunshine.
- Vargo Outdoors