Utility fog (coined by Dr. John Storrs Hall in 1993) is a hypothetical collection of tiny robots that can replicate a physical structure. As such, it is a form of self-reconfiguring modular robotics.
Hall thought of it as a nanotechnological replacement for car seatbelts. The robots would be microscopic, with extending arms reaching in several different directions, and could perform three-dimensional lattice reconfiguration. Grabbers at the ends of the arms would allow the robots (or foglets) to mechanically link to one another and share both information and energy, enabling them to act as a continuous substance with mechanical and optical properties that could be varied over a wide range. Each foglet would have substantial computing power, and would be able to communicate with its neighbors.
In the original application as a replacement for seatbelts, the swarm of robots would be widely spread-out, and the arms loose, allowing air flow between them. In the event of a collision the arms would lock into their current position, as if the air around the passengers had abruptly frozen solid. The result would be to spread any impact over the entire surface of the passenger's body.
While the foglets would be micro-scale, construction of the foglets would require full molecular nanotechnology. Each bot would be in the shape of a dodecahedron with 12 arms extending outwards. Each arm would have four degrees of freedom. When linked together the foglets would form an octet truss. The foglets' bodies would be made of aluminum oxide rather than combustible diamond to avoid creating a fuel air explosive.
Hall and his correspondents soon realised that utility fog could be manufactured en masse to occupy the entire atmosphere of a planet and replace any physical instrumentality necessary to human life. By foglets exerting concerted force an object or human could be carried from location to location. Virtual buildings could be constructed and dismantled within moments, enabling the replacement of existing cities and roads with farms and gardens. While molecular nanotech might also replace the need for biological bodies, utility fog would remain a useful peripheral with which to perform physical engineering and maintenance tasks.
In science fiction 
As early as in 1964, the idea of nanobotic swarms has been described by Stanislaw Lem in the novel The Invincible, which is set on a planet where the ecology has been taken over by nanobots. More recent science fiction novels have explored the possible consequences of the emergence of this technology, e.g., Michael Crichton's Prey (2002), or incorporated this technology as part of the story universe, e.g. Hannu Rajaniemi's The Quantum Thief (2010).
In the postcyberpunk comic series Transmetropolitan, there is a race of beings known as foglets. Through a complicated technical process, their consciousness is transferred into a cloud of billions of foglet robots—a process they see as stripping away their biological limitations and leaving them with only personal amusement. The now-vacant body is then used as fuel to jump-start the foglet. (Issue 6 page 19) They can spread themselves so thin they seem invisible, and come together as a pink cloud of dust with digital faces when they wish to be seen. (Issue 6 and beyond)
A suggestion was made by Jim Al-Khalili that the chameleonic external surface of a TARDIS could be composed of utility fog in the programme "How To Make A Tardis", broadcast as part of the nostalgic Doctor Who Night on BBC2 late in 1999.
See also 
- Programmable matter
- Self-Reconfiguring Modular Robotics
- Grey goo
- Utility Fog at Nanotech Now, many links.
- John Storrs Hall, "Utility Fog: A Universal Physical Substance," in Vision-21: Interdisciplinary Science and Engineering in the Era of Cyberspace, G. A. Landis, ed., NASA Publication CP-10129, pp. 115-126 (1993)
- Utility Fog: The Stuff that Dreams Are Made Of, Dr. J. Storrs Hall, July 5, 2001, KurzweilAI
- What I want to be when I grow up, is a cloud, J. Storrs Hall, July 6, 2001, KurzweilAI
- On Certain Aspects of Utility Fog by Dr. J. Storrs Hall
- LEGOs (TM) to the Stars The Assembler, Volume 4, Number 3 Third Quarter, 1996, Tihamer Toth-Fejel