Many animals, notably salamanders, axolotls, starfish &etc are able to regrow limbs which have been lost to accidents or disease. Although mammals can regrow various tissues (notably skin, liver and tail tips) there is no capacity to regrow limbs etc. If this could somehow be 'turned on' it would be of huge benefit to amputees etc. At present the biological mechanisms which control for regrowth are unknown.
Several versions of 'Regenerative Medicine' are currently under development - with various drugs being tested for their potential to induce regrowth. None is as yet in routine use.