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).
Bibliographer is a bibtex bibliography database editor which aims to be easy to use. Its features include linking files to your records with indexing and searching support. The interface is designed for the easy navigation of your bibliography. Double clicking a record will open the linked file.
Itzam/Sharp is a C# class library for creating and manipulating keyed-access database files containing variable-length, random access records. Information is referenced by a user-defined key value; indexes may be combined with or separate from data. Itzam/Sharp implements the Itzam engine (see Itzam/Core and Itzam/Java) in 100% managed C#. At 32K, the Itzam/Sharp engine is both small and powerful, and it works with any .NET language, including F# and Visual Basic.