An 11-year longitudinal study sponsored by the National Cancer Institute has been halted 3 years early because its findings are particularly clear and striking: finasteride, a drug currently used primarily to fight baldness and enlarged prostates, reduces the chance that a man will develop prostate cancer by 25%.
Prostate cancer afflicts 221,000 American men each year, killing 29,000, and the only treatment is to remove part or all of the prostate which often leaves the patient impotent and incontinent. Finasteride drugs, such as Propecia and Proscar -- which are already approved for sale by the FDA -- could greatly reduce the number of men afflicted with this disease, saving thousands of lives per year and millions of dollars in medical costs.
Men with a history of prostate cancer in their family are particularly at risk for developing it themselves. There appear to be few side-effects to the treatment (except maybe growing more hair).
Allen Glosson posted a good deal more information in the comments section, go read it. He points out that there are several available treatments for prostate cancer other than surgical removal, but all of these generally lead to sterility and often to impotence and incontinence.