Distributed search - multiple search engines can search segmented indexes on multiple machines.
Scales to at least 1.2 billion records, 30 million searches per day on a 15-server installation
Source code and binaries available
Administration by config file
Articles & Reviews
Sphinx - text search the Pirate Bay wayTheRegister.com: May 8 2009, by Ted Dziuba.
Describes how Sphinx replaced the MySQL search at Craigslist, cutting the server requirements in half, and runs the Pirate Bay torrent search engine. Includes some feature info and comparison with Solr. There are two very happy Sphinx users in the comments, just ignore the MySQL vs. PHP argument.
Build a custom search engine with PHP A hands-on how-to article using an example of auto-body parts, a complex MySQL table to store the data, and Sphinx
to search, especially for "OR" searches and complex queries.
PHP Search Engine Showdown
April 03, 2007
Michael Douma First page has a nice overview of site search for database-backed sites, second page has quick reviews of iSearch (1 star), MnogoSearch (3 stars), PHPDig (2 stars), RiSearch (1 star), Sphider (3 stars), Sphinx( 3 stars). and TSEP
Search Tools Consulting's principal analyst, Avi Rappoport, may be available to help you with selection, analysis, user experience, and functional search engine work. Please contact us with your questions, comments, or possible consulting discussions.