Fighting for Change
As mentioned in previous posts, I agree with the Objectivist position that to change a culture, one has to change its underlying ideas. Therefore I think the projects which ARI has launched, particularly the Objectivist Academic Center (OAC) and the high school book project, are the most important activities which one should support. However, many good, but less philosophical, people either don't see the long term importance of philosophy, or they want to fight for changes which will have more immediate effects. To those people, I would say that the two best things to fight for are legalization of drugs and privatization of the public schools. I think the latter is pretty self-explanatory, but let me add a few words about the former.
Now please don't get me wrong, I think it is immoral to do drugs, since they subvert and destroy one's mind (i.e. one's means of survival), but it is a personal choice which people must make on their own. Preventing people from acting on their choices, when such choices don't involve initiating force against others, is a violation of individual rights, and should be combated on that basis. And from a societal point of view, so many terrible and far-ranging effects come from criminalizing drugs that I think it is one of the most pressing issues extant. Among these effects are: the impetus, creation and funding of organized crime (here and in many third world countries); the distraction of law enforcement and judiciary from pursuing real crimes; the enormous cost of law enforcement, prosecution and detainment; and the ever-increasing size of the paternalistic police-state. On this last point, see the article "Spy vs. Spy" (hat tip: The Secular Foxhole).
Unfortunately, in today's climate, de-criminalizing drugs seems to be an exclusively libertarian issue (and to be clear: I don't support libertarians in any way, shape or form), so if anyone knows of a non-libertarian site dedicated to advocating the de-criminalization of drugs, please share the URL in the comments.