URN namespace registrations are maintained by IANA [1].
One well-known example is the ISBN namespace [2], where the namespace-specific string is an ISBN [3].
The term 'URI' emerged as somewhat of an abstraction over URLs and URNs [4]. People were also catching onto the fact that URNs are conceptually useful, but you can't click on them in a mainstream browser, making its out-of-the-box usability poor.
DOI is an example of a newer scheme that considered these factors extensively [5] and ultimately chose locatable URIs (=URLs) as their identifiers.