Image by ittybittiesforyou via FlickrSanta Claus machine (n.) - Whimsical nickname for a self-fueling universal constructor; essentially, a machine that can create any object or structure desired by transmuting any materials already on hand. The term was coined by the late physicist and nuclear disarmament advocate Ted Taylor. Santa Claus machines are often seen as necessary components for the creation of megastructures, as the time and materials necessary to build Dyson Spheres or Niven Rings under direct human supervision and effort is astronomically impractical. (I once did some back-of-the-napkin math on what it would take for NASA to build a Death Star, and that's a pretty clear case for why we need Santa Claus and a legion of tireless robo-elves.)
Some allegory or equivalent of the Santa Claus machine is a long-held staple of speculative fiction. Star Trek's replicators are perhaps the most famous example, though the pharaohic chemical transmuter factories from Kim Stanley Robinson's Mars trilogy also fit the bill, as do the semi-sentient household "makers" from Warren Ellis and Darick Robertson's comic series Transmetropolitan. Self-directing Santa Claus machines are also fodder for sci-fi-horror, as they may be a precursor to a gray goo outbreak. In the right hands, Santa Claus machines could lead to a post-scarcity economy (cue the Whuffie references). Paradise or apocalypse, Santa Claus machines could bring about either.
I bring it up because: Uh, Christmas Eve. Duh!