The IETF XML Registry (RFC 3688), Michael Mealling.
* This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas. *
Interesting that RDFS is explicitly named, and probably because Michael is the author and knows RDF. There isn't a centralised registry for RDF schemas - it isn't required, but is useful to locate them and find terms. The W3C doesn't do registry functions but does host files that are RDF schema. There are third party lists or services such as SchemaWeb. which allow browsing and searching of the schemas available at various places on the web.
This service is however, for schemas for IETF standards which looks like an opportunity. RDFS for vCARD, iCalendar, RFC822 email headers, ...?