Lord Jesus Christ, have Mercy on me a Sinner

The Jesus Prayer (Η Προσευχή του Ιησού) is a short, formulaic prayer esteemed and advocated within the Eastern Orthodox church:

“ Κύριε Ιησού Χριστέ, Υιέ του Θεού, ελέησόν με τον αμαρτωλόν. ”

“ Lord Jesus Christ, Son of God, have mercy on me, a sinner ”

The prayer has been widely taught and discussed throughout the history of the Eastern Churches. It is often repeated continually as a part of personal ascetic practice, its use being an integral part of the eremitic tradition of prayer known as Hesychasm (Greek: ἡσυχάζω, hesychazo, "to keep stillness"). The prayer is particularly esteemed by the spiritual fathers of this tradition (see Philokalia) as a method of opening up the heart (kardia) and bringing about the Prayer of the Heart (Καρδιακή Προσευχή). The Prayer of The Heart is considered to be the Unceasing Prayer that the apostle Paul advocates in the New Testament.

The prayer's origin is most likely the Egyptian desert, which was settled by the monastic Desert Fathers in the fifth century.
The practice of repeating the prayer continually dates back to at least the fifth century. The earliest known mention is in On Spiritual Knowledge and Discrimination of St. Diadochos of Photiki (400-ca.486), a work found in the first volume of the Philokalia. The Jesus Prayer is described in Diadochos's work in terms very similar[citation needed] to St. John Cassian's (ca.360-435) description in the Conferences 9 and 10 of the repetitive use of a passage of the Psalms. St. Diadochos ties the practice of the Jesus Prayer to the purification of the soul and teaches that repetition of the prayer produces inner peace.

The use of the Jesus Prayer is recommended in the Ladder of Divine Ascent of St. John Climacus (ca.523–606) and in the work of St. Hesychios the Priest (ca. eighth century), Pros Theodoulon, found in the first volume of the Philokalia. 
Though the Jesus Prayer has been practiced through the centuries as part of the Eastern tradition, in the twentieth century it also began to be used in some Western churches, including some Roman Catholic and Anglican churches.

From  http://en.wikipedia.org/wiki/Jesus_Prayer