A general assembler capable of producing any material object given the appropriate raw materials. This is one of the two holy grails of nanotechnology (the other being Utility Fog). With a Santa Machine one can literally scoop up piles of rotting plant material and turn it into a fine meal.

Since extremely lightweight materials could be synthesized people could have a Jetson's style fold up car (well, it would like more like evaporation or disintegration than folding, I would imagine) that slipped into their pocket between uses. Bye-bye furniture, just compile chairs as you need them and turn them into tables or statues when you don't.

Building a Santa Machine requires a few things, one of which seems unlikely to come to pass. It obviously requires a strong form of working nanotech (for the assembler). It also requires major advances in either data storage or network transmission of data to handle the (presumably) phenomenal quantities of information required for matter compilation. Finally, and most unlikely, it requires free access to an enormous database of object designs as well as free access to extremely sophisticated search tools to effectively constrain that enormous object space.

The technological concerns are all addressed by the mere existence of nanotechnology. Atomic level control will enable storage of massive quantities of data while improved optical capabilities will solve the bandwidth dilemma. However, it seems unlikely that designs, blueprints, schematics, source code, or whatever it is we have that makes an assembler spit out a filet mignon instead of a McDonald's cheeseburger will not be regarded as intellectual property of some sort with stiff social sanctions for infringement.

The only literary mention of Santa Machines that I've found is in Damien Broderick's book The Spike.