According to Google Scholar (in parentheses: total number of on-line articles and books citing the book in question):

1. Thomas Kuhn, The Structure of Scientific Revolutions (37,197)

2. John Rawls, A Theory of Justice (26,768)

3. Ronald Dworkin, Taking Rights Seriously (7,892)

4. Ludwig Wittgenstein, Philosophical Investigations (7,169)

5. Karl Popper, Conjectures and Refutations (6,516)

6. Alasdair MacIntyre, After Virtue (6,579)

7. Gilbert Ryle, The Concept of Mind (6,356)

7. John Rawls, Political Liberalism (6,352)

9. Robert Nozick, Anarchy, State, and Utopia (6,246)

9. H.L.A. Hart, The Concept of Law (6,212)

11. Richard Rorty, Philosophy and the Mirror of Nature (5,616)

12. John Searle, Speech Acts (5,387)

13. Jerry Fodor, Modularity of Mind (5,050)

14. Daniel Dennett, Consciousness Explained (4,810)

15. Karl Popper, Objective Knowledge (4,701)

Runners-up: Saul Kripke, Naming and Necessity (4,535); W.V.O. Quine, Word and Object (4,565); Paul Feyerabend, Against Method (4,420); Richard Rorty, Contingency, Irony and Solidarity (4,011); Charles Taylor, Sources of the Self (3,233); Jerry Fodor, The Language of Thought (3,292); Carl Hempel, Aspects of Scientific Explanation (3,137); David Chalmers, The Conscious Mind (3,065), Daniel Dennett, The Intentional Stance (2,985); Derek Parfit, Reasons and Persons (2,972).

Since Google Scholar put law reviews on-line recently, that gives a big advantage to books in political and legal philosophy. In addition, any philosphical work of interest to psychologists or linguists does very well, since those two disciplines have substantial on-line presences, more so than other fields.

UPDATE: I'm opening comments, so some of the philosophers with whom I've been corresponding about how to define what's included and what's not, as well as those with other corrections/addenda to the list, can post their thoughts. Full signature and valid e-mail please!