“is it legal to swipe in a friend after waiting 18 minutes unlimited metrocard” (6/9/09)

Yes! This is important to understand, and was explained in a recent Times article:

Selling a swipe on a MetroCard is illegal and can get you arrested. Bending a MetroCard’s magnetic strip to fool the turnstile into letting you through is also a form of theft.

But letting a friend or a relative use your unlimited-ride MetroCard when you are not using it is perfectly legal, as long as you don’t charge for it, said Paul J. Fleuranges, an authority spokesman. (The card allows only one entry every 18 minutes.)

Mr. Fleuranges said it is also legal to help out a stranger who asks you, as a favor, to swipe him through a turnstile free as you are leaving a subway station — although it certainly deprives New York City Transit of a fare.