One thing people don't consider (but should) is Harris Stay Brite solder. I like JB Weld and use it all the time, but I can promise you this will repair cast iron if you clean the crack, use their flux and are able to heat the area to about 450 degrees. Most of the time you can do this with a MAPP or Propane torch. I've repaired plenty of cast iron, mild steel, even stainless steel with it. Note: it isn't cheap but I'll bet you'll find uses for it the rest of your life. Also it is lead-free (silver bearing solder) so you can use it for plumbing.
Jon
