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).
Search::Xapian is a Perl XS frontend to the Xapian C++ search library. It is a fairly complete wrapper: most features of the Xapian library are made available for use from Perl. Xapian 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 as well as a rich set of boolean query operators. It's fast and scalable to hundreds of millions of documents.
Emdros is a corpus query system for storing and searching linguistically annotated text. It is very generic, supporting almost any kind of annotation from almost any linguistic theory. All linguistic levels of analysis are supported, including phonology, morphology, the lexical level, syntax, and discourse. The core libraries act as a middleware layer between a client and an underlying SQL database. MySQL, PostgreSQL, and SQLite are supported.
Dowser is a Web research and archiving tool that clusters results from search engines, associates words that appear in previous searches, and keeps a local cache of all the results you click on in a searchable database along with summaries and links to related information. It helps you to keep track of what you find, with no advertising.