I've been working on a chapter describing the Tragedy of the Commons for the forthcoming Encyclopedia of Natural Resource Economics (I'm also writing one on Coastal Resource Economics, more about that one soon). Hardin discusses the tragedy of depletion as an inevitable result when natural resources are rival and non-excludable, noting that self-interested users will tend to ignore the social costs and focus on maximizing personal benefits. To solve the problem, Hardin called for privatization of the commons or government control via rationing or taxation. We see many modern day examples of this tragedy, from over-fishing to atmospheric pollution. As I've noted on more than one occasion, all of us can be seen as the herdsmen of our day in one way or another.
It's important to point out that Hardin's tragedy does not always unfold as he described. Under certain conditions, collective action (group management) can be an effective means of achieving environmentally sustainable and economically efficient outcomes for common pool resources. Elinor Ostrom (winner of the 2009 Nobel Prize in Economics) was a leading researcher in this area.
Here is an excellent summary of Ostrom's work.
Read more about Ostrom's work here and here.
A similar notion is provided by this interesting new sharing site.
Lots of reading on the Tragedy here at Science.