Vespucci is a proposed protocol that uses the Bitcoin blockchain for decentralized application discovery.
Although there have been other proposals on this topic, Vespucci is to my knowledge the first one that does not require the entire blockchain to be downloaded. This results in a massive performance improvement.
In order for an application to join a P2P network, the application must somehow find the IP address of one or more peers to connect to. This is called bootstrapping, and it can be difficult. BitTorrent and Bitcoin clients typically include hard-coded addresses of bootstrapping nodes, long-lived server nodes. This works well for established P2P networks with multiple bootstrap nodes like Bitcoin and BitTorrent. It doesn't work well for new and/or small P2P networks, because a small network might only have a single bootstrap node, creating a single point of failure.