Topic: Wax printing?
I'd probably never try this, but I wonder if it is possible to feed beeswax or paraffin (probably already melted) through an extruder and cool it off enough as it is extruding to solidify? (No doubt a completely different kind of extruder would be required).
Seems like wax would be really easy to reuse compared to ABS since you could melt and filter it as many times as you needed to while getting the model just right. (Or even go with lost wax casting to get the part in metal).
Naturally, you'd also have to worry a lot about wax running everywhere and catching on fire (one of the reasons I doubt I'd try it :-).