I get a kick out of making new tools from old parts. I keep all my old files and chisels to turn them into something useful around the shop. The same goes for hacksaw blades. I turned a dull one into an excellent single-bevel marking knife.
I prefer a single-bevel to a double-bevel marking knife for accurately drawing a square line. (If you lean a double-bevel knife the wrong way, the line wanders away from the square.) To make this single-bevel knife, I clamped the old hacksaw blade in my metal-working vise, snapped off a 6-in.-long section and ground down the teeth. I flattened one side of the working end on sandpaper taped to my tablesaw, then curved and sharpened it with a 100-grit wheel on my grinder. A curved end with a long, continuous edge stays sharp longer than a pointed end. I attached the handles with epoxy.