Undifferentiated cells which, when they divide, can generate any other type of cell found in the body. These cells hold the best promise to growing replacement organs. There are ethical issues when using stem cells since at this time the only way to aquire them is via fertilized embryos less than a week old.
Update: recent research has it that
Edinburgh's stem cells might be overtaken by easier to get hold of
somatic cells. It seems that developmental states are much more plastic than previously thought. That is, your bone cell doesn't have to be stuck as a bone
cell all its life, but can be prompted into changing course. This involves plying the cell with
growth factors and other mystic substances to activate dormant pathways. Since this does not require embryonic tissue, it alleviates some
ethical issues - and it might be cheaper too.
First part is
Shafik's writeup, cruelly nuked in the name of efficiency. Oh, and slightly edited in the name of grammar...