Shanti Mantras are found in Upanishads, where they are invoked in the beginning of some topics of Upanishads. They are supposed to calm the mind of reciter and environment around him/her. Reciting them is also believed to be removing any obstacles for the task being started...
"dyauh shantih antariksham shantih prithvi shantih rapah shantih"
May peace radiate there in the whole sky as well as in the vast ethereal space everywhere. May peace reign all over this earth, in water
"oshadhayah shantih vanaspatayah shantih vish.ve.devah shantih"
and in all herbs, trees and creepers.May peace flow over the whole universe.
"brahma shantih sarvam shantih shanti-reva shantih saa maa shanti-re.dhi"
May peace be in the Supreme Being Brahman.And may there always exist in all peace and peace alone. (translation source : wikipedia )