crack in steel frame = weld and it's good to go (if it's from rust you might thoroughly check everything for other affected zones - with ultrasounds to check wall thickness and major cracks and penetrating fluids for revealing surface cracks) - depends of the condition, if you have to weld a lot, consider a frame-builder to replace some tubing, but really it might be cheaper a new frame.

crack in aluminum frame = pretentious weld and some heat treatment (in some occasions), but if it's from fatigue of aluminum .. well the frame lived it's life, Aluminum builds a lot of internal stress and if cracks show up then it's more likely to get a lot more in short time. - lifespan of Aluminum frames is limited by fatigue. I'd say at most 20-30years of riding (as opposed to steel that might hold up for your grandchildren.. when they are old)

crack in carbon = no can do! you really have to get another frame