The User Directory is a service that will allow Tox users to add their Tox ID for easy sharing. Instead of having to remember their pubkey, they can simply paste it into a field, along with their display name of choice, as well as URL of choice. For instance, someone can add the Tox ID "A5E2D6947CDDD7E9CA37DF1ADF3C3B375AFF3AAB660FBDC0FE1EC599A43782430E84181F9997" and have it shortened to http://mydomain.com/MyUsername that will display a page with their username, Tox ID, as well as a Friend Me button using the tox:// URL scheme.
Anyone can be able to run one, and of course this is still in extremely early alpha. There is already an API for clients to intergrate with for automatic addition, but documentation has yet to be made.