In ancient Egypt, the Eye of Horus (also called the udjat or utchat or "sound eye") was a powerful symbol and protective amulet. During his battle with Set, Horus’ eye was plucked out and torn apart. But Thoth was able to piece it back together, adding a little bit of magic to it to make up for a tiny piece that was missing. As the regenerated eye of the falcon god Horus, it was worn as a sacred symbol to ensure good health. Among modern tattoos that use ancient Egyptian motifs, the Eye of Horus ranks near the top, along with the Ank.
Tibetan Buddhists believe that saying the mantra (prayer), Om Mani Padme Hum, out loud or silently to oneself, invokes the powerful benevolent attention and blessings of Chenrezig, the embodiment of compassion. Viewing the written form of the mantra is said to have the same effect -- it is often carved into stones, like the one pictured above, and placed where people can see them.