OpenText Search Server is one of the fastest and most reliable full-text search engines on the market. Its core technology is designed to support information repositories in any existing language and can be used in any circumstance. It is scalable to multi-terabyte databases and thousands of users.
LINK: http://www.opentext.com/
Dtsearch Engine provides simple to use but very powerful tools which create and maintain full text indexes of documents. Instantly Search Terabytes of Text.
LINK: http://www.dtsearch.com/
Dieselpoint provides advanced full-text search with data navigation capability. Dieselpoint Search is a Java search and navigation system for enterprise data, including large-scale product catalogs, website search, document search.
LINK: http://dieselpoint.com/
Yacy is an opensource search engine, fully decentralized and using Peer to Peer. 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.
LINK: http://search.yacy.net/