Blackburn, Simon. Think: A Compelling Introduction to Philosophy (Oxford: Oxford University Press, 1999)

Plato, Meno, Theaetetus (the Burnyeat edition has a brilliant introductory discussion of how the dialogue relates to contemporary philosophy, so is a great introduction to the discipline). I would also say these are the most important works of Plato to read apart from the Republic for philosophers.

Aristotle, Posterior Analytics

Descartes - Meditations (introduces radical scepticism into the philosophical mix; the first few chapters are the most important)

Hume, David - An Enquiry Concerning Human Understanding, Section 4 (problems of induction)

Gettier - Is Justified True Belief Knowledge? Analysis 23: 121-3 (a 2-page-long classic that changed the course of epistemology!)

Nozick (1981), part 3 of Philosophical Explanations (introduces reliabilism)

Epistemology: an Anthology (Blackwell) as a whole is excellent - it is a collection of the main important papers from the 20th Century presented in a very clear way.