Black Cohosh is a herb native to North America that has traditionally been used for cognitive and inflammatory conditions, but has grown in popularity due to it's ability to treat vasomotor symptoms of menopause; primarily hot flashes and night sweats. It is one of the most popular and highest sold supplements in the Western world (10th place in 2008), according to some surveys.
Studies on the matter are highly mixed. The larger body of evidence favors the efficacy of Black Cohosh for treatment of vasomotor symptoms but consists largely of unblinded studies; as the placebo effect can reduce menopausal complaints, blinding is needed. Efficacy has been demonstrated with blinded studies on Black Cohosh as well, but many of them are confounded with consumption of other compounds. A few blinded studies on Black Cohosh without any other compounds have been conducted, and are basically split right down the middle on efficacy if not favoring 'no significant effects' a little bit more due to quality of data and sample size.
Beyond the questionable efficacy, Black Cohosh appears to be safe. It is non-estrogenic (despite being thought to influence estrogen in the past) and may act centrally (in the brain) via serotonin, dopamine or opioids. Stomach upset has been reported and seems to be attributable to Black Cohosh in some people, but reports of liver toxicity do not appear to be related to the Black Cohosh herb. These reports do exist, but they cannot be linked to Black Cohosh logically.