The Fremen are the hardened, resourceful native people of Arrakis in Frank Herbert's hugely successful series of Dune books.
They live out in the desert, making homes in large rock bluffs called a Sietch. Each Sietch is self-sufficent, capturing and storing the water they need to survive. They wear stillsuits, which reclaim nearly all the water they sweat or exhale so that they lose less than a thimble-full a day. Because of their constant exposure to the Spice, their eyes turn completely blue.
More interesting than their physical adaptations, though, are their cultural adaptations. Because of the sacredness of water, a man is seen to be holding water for the tribe in his flesh, so if a man dies, the water is extracted and given to his survivors. If a man is killed in an honorable duel, his water and all his possessions (including his wife) are the possessions of the victorious duelist. To get around the vast and treacherous deserts of Arrakis, the Fremen have learned that one must walk without rhythm so as not to attract a sandworm; Sandworms, however, can be an effective mode of transport for the wily Fremen, they use devices called thumpers to call Sandworms, then use maker hooks to climb onto the back of the segmented worm, riding it like crazy sand cowboys. Being able to do this is a major rite of passage for a Fremen male, who is expected to do it by about age 14.
One of the most sacred objects to the Fremen is the Crysknife, a knife made from the tooth of a Sandworm. Anyone who has seen a Crysknife is not allowed off the planet alive, it's such a precious secret.
A bit of origin: The Fremen, while seen as native to Arrakis, are not originally from there. They were driven from their homeworld and persecuted for millenia. Called the Zensunni wanderers, they were followers of a religious sect that emphasized the mystical and reverting to the ways of the fathers. They wandered the inhospitable planets of their galaxy (including Solusa Secondus!) until they finally settled on Arrakis.
The Fremen were meddled with many years before the events of Dune by the Bene Gesserit, implanting all sorts of myths about things that would come to pass in the events of Dune, making things very convenient for Paul Atreides and the Lady Jessica in their flight from the Harkonnen.
The Fremen are also awesome in battle. They can take out Sardaukar 1:10 against.
Obviously, this is a cursory overview of the Fremen, and if you want to learn more about them, it's a google away! (Heck, you can even find the full text of all the Dune books online if you look hard enough!)
Source: My copy of Dune, paperback, from 1965, published by Ace, and the Wikipedia. Thanks to Unperson for suggesting discussing their origins and the crysknife.