According to the National Archives, as of 2014, the most recently passed amendment to the U.S. Constitution was the 27th. It was ratified on May 7, 1992.
The National Constitution Center reports that the 27th amendment prevents Congress from giving themselves pay raises while in session. Any raises granted must be postponed until the following session. The amendment was designed to make Congress more careful about pay increases, since no direct benefit is seen immediately. Although the amendment became law in 1992, it was originally proposed to Congress by James Madison in 1789. In the latter part of the 20th century, many citizens were disturbed by congressional pay hikes, and three-fourths of the states finally passed the measure.