http://yacy.net/
YaCy is a scalable web search engine with an integrated web crawler and content analysis and managements functions. One YaCy installation can store more than 20 million documents, but in a community of search peers YaCy can provide a search index of unlimited size.
Peer-to-Peer Index Sharing
We implemented a index sharing method, that works similar to peer-to-peer file sharing. YaCy shares web index information with other peers and is therefore resistant against censoring the same way as P2P file-sharing is resistant against deletion.