P2P-Based Search Engine
Search engines based on P2P-methods, also called 'collaborative crawling and indexing'
Definition
"In P2P search (a.k.a. distributed search), each individual connected to the network serves its local index as a source of search. Instead of having a central company and a central server, each participant of the network is a search repository. Since we are talking about web indexing and web searching, a user's internet cache might be their contribution to the search database. When they execute a query, firstly their local system is queried; than if the results are not satisfactory, the next peer is queried, and so on. The difficulty is the selection of good peers to provide satisfactory results." (http://www.readwriteweb.com/archives/p2p_potential_future_applications.php)
Examples
The Minerva Project
URL = http://www.mpi-inf.mpg.de/departments/d5/software/minerva/index.html
""Each peer is considered autonomous and has its own local search engine with a crawler and a corresponding local index. Peers share their local indexes (or specific fragments of local indexes) by posting meta-information into the P2P network. This meta-information contains compact statistics and quality-of-service information, and effectively forms a global directory. However, this directory is implemented in a completely decentralized and largely self-organizing manner." (cited in http://www.readwriteweb.com/archives/p2p_potential_future_applications.php)
More examples
Open-Search, at http://www.open-search.net/Opensearch/OpenSearchFoundation
GPU P2P Crawler, open-source based
Faroo, at http://www.faroo.com/