Most copies of the Septuagint contain one additional, relatively short psalm beyond the 150 found in the Masoretic text. This psalm is colloquially called Psalm 151. In actuality the title given to this psalm in the Septuagint indicates that it is supernumerary, and no number is affixed to it.

The title of the psalm indicates that it was written by King David after his battle with Goliath. The text (full English text here http://www.xs4all.nl/~pboot/Daphne/Pers.htm , scroll down to find it) expresses how David was the least of his brothers, and yet God chose him to be anointed king. It goes on to commemorate how David killed Goliath with the Philistine's own sword.

The psalm is considered by scholars to be an addition by the compilers or translators of the Septuagint. The Orthodox use it in their Bible, while Catholics, Protestants and Jews consider it apocryphal.