Net::Z3950::SimpleServer is a Perl module which implements the server side of the Z39.50 (information retrieval) protocol. It hides the complexity of network exchanges, packet serialization, and session handling. You are required only to implement simple callbacks to support searching and record retrieval. It is the basis of the "Zoogle" project, which is a Z39.50 gateway to the Google web index.
XMLDB uses an RDBMS to persist arbitrary XML documents. Due to its storage mechanism, searching for and recalling documents is extremely quick. You can also perform XSL translation on documents with surprising speed. The library can be used in any program to store libxml2 documents. A PHP module is also included, making XMLDB into a complete three-tier Web application development suite.
libextractor is a library used to extract meta-data from files of arbitrary type. It is designed to use helper-libraries to perform the actual extraction, and to be trivially extendable by linking against external extractors for additional file types. The goal is to provide developers of file-sharing networks, file managers, and WWW-indexing bots with a universal library to obtain meta-data about files. It includes a shell-command and bindings for Java (JNI) and Python.
Pyndex is a simple and fast full-text indexer implemented in Python. Pyndex also includes an easy to use Bayesian classifier. It uses Metakit as its storage back-end. It works well for quickly adding a search feature to an application, and is also well suited to in-memory indexing and searching. It can handle phrase queries. It performs best in applications involving a few thousand documents, but its scaling is mostly limited by available memory.
SearchAssist is a simple but practical search engine application that uses a ternary search tree. It uses Java's dynamic loading feature to make the search engine highly customizable, and uses takes Mozilla bookmarks as input. A Swing UI allows users to enter search words and view the results.
Xapian is a search engine library, scalable to collections containing hundreds of millions of documents. It's written in C++ with bindings for Perl, Python, PHP, Java, Tcl, C#, Ruby, and Lua. It is a highly adaptable toolkit that allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also a rich set of boolean query operators. Omega is a Web search application built upon the Xapian library. It can index a Web server's document tree (including HTML, PDF, OpenOffice, MS Word/Excel/Powerpoint/Works, WordPerfect, RTF, PS, etc.), or data exported from arbitrary sources (e.g. SQL databases).