Om Mani Padme Hum

The mantra Om Mani Padme Hum (literally: "Aum, to the Jewel in the Lotus, hum) is recited by Tibetan Buddhists to invoke Chenrezi, the Bodhisattva of Compassion. Repeating this mantra accumulates merit and eases negative karma; meditating upon it is believed to purify the mind and body.

http://en.wikipedia.org/wiki/Om_mani_padme_hum
http://dharma-haven.org/tibetan/meaning-of-om-mani-padme-hung.htm
http://www.circle-of-light.com/Mantras/om-mantra.html
http://altreligion.about.com/library/glossary/symbols/bldefsmani.htm