From sewing machine to scroll saw, check out Instructables member bongodrummer's in depth HowTo.
"...Old sewing machines, that are actually really rather well made, are often thrown out by people who do not have the time or inclination to use or maintain them...
Converting a sewing machine shouldn't be all that difficult - hence the idea. You have a needle that goes up and down at a speed fast enough to drive the saw blade, a work area with a section underneath to keep tension on the blade... What could be simpler? All you need are some basic hand tools, some epoxy, and an old unused sewing machine."
If it's as easy as it sounds, I find the idea pretty ingenious.