This article talks about DCA, or Dichloroacetic Acid and its interaction with cancer cells. Additionally, it sounds like the observation of this interaction has provided understanding of the key role mitochondria play in cancer cells. I think that this new understanding is fairly exciting.
According to the Wikipedia article linked to above:
"Although dichloroacetate "has actually been used safely in humans for decades" its therapeutic use can cause pain, numbness and gait disturbances in some patients. A clinical trial where DCA was given to patients of MELAS (a form of genetically inherited lactic acidosis) at 25 mg/kg/day indicated peripheral nerve toxicity. Dichloroacetate can also have anxiolytic or sedative effects."
One of the more fascinating aspects of this discovery, is that DCA is unencumbered by any patents.