Slate has an article on social philosopher Nozick and libertarianism. I'm not totally convinced that he can be dismissed this readily, but there it is. His Wikipedia entry makes some bald claims that he did renounce his raw libertarianism of the early 1970s by the end of the 1980s in a book...
I've been vaguely curious about abstract notions of "justice" since I bought Amartya Sen's recent book on the topic, where he takes John Rawls to task, I think. It is still on the shelf after a year or two, unfortunately. As far as I understand it, Sen's "capabilities" incorporate both talent and context, that is, a person's talents are a bit useless if she is not in a position to use them, and as society plays a large role in providing (mediating) opportunities, social obligations arise as practical necessities. This seems pretty obvious, and Quiggin beats libertarians over the head with more-or-less this point. Utopianism ahoy...
Update: Metcalf posted a rejoinder to all those who doubt him.