Sphider
- A lightweight search engine in PHP. Includes details of features, documentation, support forum, and download. [GPL]
- http://www.sphider.eu/

Nutch
- Effort to implement a prototype of an open source web-search engine.
- http://lucene.apache.org/nutch/

OpenWebSpider
- An open source web spider and search engine. Includes demo, source code and screenshots.
- http://www.openwebspider.org/

Open Source Search
- Resources, ideas and news site for developing next generation open source search engines. Includes relevant source libraries and white papers.
- http://www.opensourcesearch.org/

BDDBot
- A web robot, search engine and web server written in Java and available under GPL. Includes related resources. [Project no longer actively updated]
- http://www.twmacinta.com/bddbot/

Egothor
- A cross-platform, full-featured text search engine written entirely in Java. It can be configured as a standalone engine, metasearcher, peer-to-peer HUB or used as a class library.
- http://www.egothor.org/

JXTA Search
- A distributed search system, designed for peer to peer networks and web sites.
- http://search.jxta.org/

DataparkSearch Engine Tool
- Open source search engine tool released under GPL and designed to organize search within a website, group of websites, intranet or local system.
- http://www.dataparksearch.org/

Search Tools Listing: Open Source Search Engines
- A listing of open source indexers, including links to related articles.
- http://www.searchtools.com/tools/tools-opensource.html

KDE GUISearch
- A tool for finding code by looking at the applications' GUI text messages (e.g., "Undo") and returning associated callbacks/slots (e.g., slotUndo()). Allows searching the KDE project CVS repository as a live demonstration.
- http://guisearch.sourceforge.net/