Larry Moran over at the Sandwalk and PZ Myers at Pharyngula both encourage us to read This Week's Citation Classic: Gould and Lewontin's The Spandrels of San Marco and the Panglossian Paradigm: a Critique of the Adaptationist Programme.
I have just re-read the paper, and can confirm that, 28 years after its original publication, it is as relevant as ever. The one-line summary:
One must not confuse the fact that a structure is used in some way […] with the primary evolutionary reason for its existence and conformation.
But don't trust to one-line summaries; the paper is well worth 20 minutes of your time.
What else are you going to do on a Sunday morning? Everyone else is at church!