Tritium is a radioactive isotope of Hydrogen. It doesn't pose a danger to the human body since the beta radiation is blocked by the skin. These properties allow it to be used to illuminate anything from street signs to wristwatch hands. The main advantage of using Tritium is longevity - illuminations usually last for more than a decade.
A novelty item that uses Tritium is the Tritium light keychain. It can be bought around the Internet. See video below for a review of such a keychain.