Judicial Review
The power of the Supreme Court to declare a law unconstitutional. This power is what makes the judicial branch perhaps the most powerful branch of the American government, as it involves overturning a legislative decision of Congress, and there is no way to override a Supreme Court decision unless a Constitutional Amendment is made (this is a very long and difficult process).
Judicial review is never specifically mentioned in the Constitution, but instead is perhaps implied in Article III:
"The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority."
Judicial review is, however, mentioned by Alexander Hamilton in The Federalist Papers, and was established in the landmark case Marbury v. Madison in 1803 by the ruling of Chief Justice John Marshall. It is now a well established power of the Supreme Court, despite its how debatable its Constitutionality may be.
Resources
US Government and Politics by Paul Soifer