I'm not sure if the most interesting thing I've seen done with 3d printers has actually been done yet or is still a concept but there was an article a while back discussing 3d printers for use in space exploration (moon/mars landing, etc.) Basically arrive on site with limited supplies and equipment other than a printer, then use said printer along with materials found on site to produce anything that was needed. Pretty awesome application as the cost of firing things into space is massive so the ability to produce whatever was needed on site rather than having to ferry massive payloads into space would hugely reduce the cost and logistical complexity of colonizing elsewhere in the solar system

My dream 3d printer would combine additive and subtractive methods of fabrication. It would function much like a modern CNC machining center with multiple tools to be used as needed: multiple FDM nozzles (sizes, materials, colors, etc), cutting tools for subtractive fabrication, and possibly a mirror and laser setup to do SLS as well. Each process has it's own benefits and drawbacks so combining the three would allow them to be used in sync to produce otherwise difficult or impossible geometries and properties.