The full text of the serenity prayer reads as follows:
God, give us grace to accept with serenity the things that cannot be changed,
courage to change the things which should be changed,
and the wisdom to distinguish the one from the other.
Living one day at a time,
Enjoying one moment at a time,
Accepting hardship as a pathway to peace,
Taking, as Jesus did, this sinful world as it is,
Not as I would have it,
Trusting that You will make all things right,
If I surrender to Your will,
So that I may be reasonably happy in this life,
And supremely happy with You forever in the next.
Amen.
This prayer was allegedly written by Reinhold Niebuhr in 1934. He was a theologian and a preacher, as well as a defender of human rights, a pacifist and a lecturer. Although he wrote it and then thought little of it, Alcoholics Anonymous published the prayer in one of its pamphlets and it's fame grew, it now being part of the twelve step programme for recovering from alcoholism. Despite a rather prolific career and many books on life, love, justice, peace and religion, the Serenity Prayer is Niebuhr's most famous work.
Other sources suggest that it was in fact Friedrich Christoph Oetinger, a German deacon, who wrote the opening three lines to the prayer in the 18th century, but Niebuhr took it from the German, translated and embellished it into the form more commonly used today.
http://www.aabibliography.com/neibuhr.html
http://www.geocities.com/~spiritwalk/serenityprayer.htm
http://manetheren.cl.msu.edu/~vanhoose/quotes/0005.html