YaCy is a free search engine that anyone can use to build a search portal for their intranet or to help search the public internet. When contributing to the world-wide peer network, the scale of YaCy is limited only by the number of users in the world and can index billions of web pages. It is fully decentralized, all users of the search engine network are equal, the network does not store user search requests and it is not possible for anyone to censor the content of the shared index. We want to achieve freedom of information through a free, distributed web search which is powered by the world's users.
Imagine if, rather than relying on the proprietary software of a large professional search engine operator, your search engine was run by many private computers which aren't under the control of any one company or individual. Well, that's what YaCy does! The resulting decentralized web search currently has about 1.4 billion documents in its index (and growing - download and install YaCy to help out!) and more than 600 peer operators contribute each month. About 130,000 search queries are performed with this network each day.
The installation takes only three minutes. Just download the release, decompress the package and run the start script. On linux you need OpenJDK7. You don't need to install external databases or a web server, everything is already included in YaCy.