Studies have shown that injecting cannabinoids into mice slowed the growth of three kinds of cancers and increased their life expectancy up to 36%.
http://www.alternet.org/story.html?StoryID=9257

I have also read that those who smoke marijuana and tobacco tend to live longer than those who smoke just tobacco. It would make sense as marijuana smoke acts as an expectorant and cannabinoids are powerful anti-oxidants, but at the moment I cannot provide you with any case studies.