Of all the things I saw at the Outdoor Retailers Expo, I think that this was the one that hit me the most with the feeling "Why didn't I think of that". Very simple idea, cheap, and super cool. The Firefly by Guyot Designs is a waterproof LED light that fits into the lid of any Nalgene sized bottle. When filled with water and tipped upside down, you now have turned your water bottle into a lantern. I saw one at the show, and it was very impressive. I am sure they don't put out a ton of light, but I dare you to find a more compact lantern. Available from Guyot Designs for $21.
They also have a very nice shorty insulated bottle for $20.