How interesting - a DNS based system for discovering several URI based services associated with a telephone number ENUM
From ENUM.org :
... it is the mapping of a telephone number from the PSTN to Internet services -- telephone number in, URL out
From RFC 3761 :
The result of the ENUM query is a series of DNS NAPTR resource records [RFC2915] which can be used to contact a resource (e.g.URI) associated with that number.