I've used an old soldering iron a number of times (wear a respirator and be in a well ventilated area, of course). I also recently came across a styro knife (I think it might be called a styrocut???) in some of the dark nether regions of our storage area. It seems like it'd be a relatively cheap device, although I would have no idea where to get one, perhaps some place that works with insulation ?
Also don't forget about the 3,000 potential uses of a Dremel rotary tool.
Demand Products makes a very useful tool with interchangeable knives. They are of course, not home grown but do very nice work. www.demandproducts.com
I have used an electric carving knife on occasion as well to good effect.
I have heard of folk who have fashioned knives from small 110v heater elements as well. [similar to broiler elements]
If you want to cut molding and balustrades, you really want a hot wire cutter instead of a hot knife. A former student who works a lot in the plaster shops in LA doing foam carving for film showed me a simple HWC consisting of nichrome wire, a household rotary dimmer switch, heater cord and some miscellaneous ABS pipe and stainless steel bolts. Cheesy looking thing but amazingly effective for about $20.
VENTILATION, VENTILATION, VENTILATION!