YaCy

YaCy is a complete search engine application: a server that hosts a search index, a web interface for searching and creating the index, and a production-ready crawler with a scheduler to keep the index fresh. It can run as a node in a distributed peer-to-peer network or as a private search portal.

YaCy search portals can also be placed inside an intranet, where the README says they can replace commercial enterprise search products, and a network scanner helps discover HTTP, FTP and SMB servers to index. The project was created with privacy as a primary motivation, and running a personal search engine avoids handing queries to a central provider. It is written in Java and tagged for decentralized search and privacy protection.

The repository license is shown as 'Other' on GitHub, reflecting the GPL family of licenses, so check the terms. YaCy is self-hosted on your own machine or server. It suits privacy-minded users, organizations that need intranet search and communities that want a decentralized alternative to commercial search.