Researchers led by Evangelos Michelakis and Kenn Petruk at the University of Alberta have published a study showing that dichloroacetate (DCA) is effective for treatment of certain brain cancers (see here, and also this recent Globe and Mail article). Three years ago, the same team showed that DCA is effective for treating a variety of cancers in vitro and in animals, which was subsequently replicated by other teams, but this is the first result of its kind in a human study.
Research into DCA has been funded primarily by donations, because the drug cannot be patented (it’s been in the public domain since at least 1864), and therefore pharmaceutical companies are not interested in funding studies.
The drug works by disrupting cancer cell metabolism, and so there is hope that the same sort of mechanism (if not the same drug) might work for other cancers, too.
However, the excitement (and desperation) of some cancer patients has led to criminals preying upon them (see the Globe and Mail article linked above). Alas. The researchers themselves have urged caution, and recommend that people wait until clinical trials have run their course, after which dosages and side effects will be more clear.
Part of the promise of DCA is that side effects are relatively few. However, some samples of DCA circulating via internet are tainted by impurities that render them dangerous. There are also cases in which DCA is counterproductive, so authorities are strongly advising against self-administration.
Update (31 December 2011): There is an informative review of the potential of DCA at Pharyngula, with many interesting and useful comments included.