As of January, 2012, this site is no longer being updated, due to work and health issues
Open source search engines allow participants to make changes and contribute to the improvement of the software. They are generally free and use the GPL or other open source licensing schemes. In most cases, anyone can use the software on a site or incorporate it in a product, but must share improvements and additional functionality with the other source users. Technical support is generally only available from other users and developers via mailing list or online conferencing, or by paying consultants.
Note that these search engines generally require all options to be set using command lines or configuration files, rather than interactive browser-based graphic interfaces. Changes are often done on the server, requiring root access and passwords.
Free Search Engine Comparison by Yiling Chen, revised April 2006, compares Alkaline, Fluid Dynamics, ht://Dig, Juggernautsearch, mnoGoSearch, Perlfect, SWISH-E, Webinator, and Webglimpse, concentrating on indexing, query processing, retrieval, and relevance.
Comparing Open Source Indexers Infomotions Musings; May 29, 2001 by Eric Lease Morgan
Describes the history and features of eight open-source search engines, freeWAIS-sf (aging code and hard to install, but good for searching email and public domain etexts); Harvest (powerful gathering features for frequently-changing data stores, good with structured documents); ht://Dig (tricky to configure, no phrase searching, automatic stemming and match word highlighting); Isearch (weak documentation and support, easy to install, dated interface, Z39.50 support); MPS Information Server (zippy indexing of both text and structured data, Z39.50 support, Perl API, limited documentation); SWISH-E (simple to install engine, CGIs in Perl and PHP still beta, good for HTML pages, recognizes new META tags, sorts results by field; WebGlimpse (easy to install and configure, requires commercial version for customized output); Yaz/Zebra (mainly Z39.50, no Perl API, mainly a toolkit to index and respond to distributed client queries). Article also points out that chaotic information is less than helpful and encourages organization, structure and vocabulary control.
SourceForge Indexing/Search List
Listings of open-source search engine projects at SourceForge, with notes about status, language and current activities.