thread-based email index, search and tagging.

Notmuch is a system for indexing, searching, reading, and tagging
large collections of email messages in maildir or mh format. It uses
the Xapian library to provide fast, full-text search with a convenient
search syntax.

• Notmuch @ GitHub.

Hexagonal hierarchical geospatial indexing system.

H3 is a geospatial indexing system using a hexagonal grid that can be (approximately) subdivided into finer and finer hexagonal grids, combining the benefits of a hexagonal grid with S2's hierarchical subdivisions.

• H3 @ GitHub.

The Apache Tika toolkit detects and extracts metadata and text from over a thousand different file types (such as PPT, XLS, and PDF).

All of these file types can be parsed through a single interface, making Tika useful for search engine indexing, content analysis, translation, and much more.

• Tika @ GitHub.

Sphider is a lightweight web spider and search engine written in PHP, using MySQL as its back end database. It is a great tool for adding search functionality to your web site or building your custom search engine. Sphider is small, easy to set up and modify, and is used in thousands of websites across the world.