ॐ मणि पद्मे हूँ
Om mani padme hum

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

"The mantra Om Mani Päme Hum is easy to say yet quite powerful, because it contains the essence of the entire teaching. When you say the first syllable Om it is blessed to help you achieve perfection in the practice of generosity, Ma helps perfect the practice of pure ethics, and Ni helps achieve perfection in the practice of tolerance and patience. Pä, the fourth syllable, helps to achieve perfection of perseverance, Me helps achieve perfection in the practice of concentration, and the final sixth syllable Hum helps achieve perfection in the practice of wisdom."