Do you mean like FreeNet?
"This document specifies Metalink/HTTP: Mirrors and Cryptographic Hashes in HTTP Headers, a different way to get information that is usually contained in the Metalink XML-based download description format. Metalink/HTTP describes multiple download locations (mirrors), Peer-to-Peer, cryptographic hashes, digital signatures, and other information using existing standards for HTTP headers. Clients can transparently use this information to make file transfers more robust and reliable."