Southwest Indians are supposed to have eaten chia seeds on long runs when they started to get tired, and in Born to Run Christopher McDougall discusses the chia smoothies used by the Tarahumara of Chihuahua for the same reason. Yes, chia, the seeds for the stuff that grows on chia pets.
It pays to be skeptical of these claims (and the great blog Science of Running can help you with that). People always want a magic fix for things like this, and unfortunately most of it is nonsense being peddled by a profit motive - though there's nothing wrong with profit, unless you're B.S.-ing people about your product. And I wanted to try it out, so I decided to do a small and admittedly poorly-controlled study: I carried chia seeds with me, and started eating them an hour into my run. I did this five times, in flat to slightly-hilly runs, at temperatures ranging from 55 to 65 F. I could feel no difference in fatigue versus previous runs under the same conditions, and my times on the same routes were the same.
If you want more, then read this paper in the Journal of Strength and Conditioning Research, which reported that chia seeds showed no statistical advantage over any other form of carb loading.