"Terrier is a highly flexible, efficient, and effective open source search engine, readily deployable on large-scale collections of documents. Terrier implements state-of-the-art indexing and retrieval functionalities, and provides an ideal platform for the rapid development and evaluation of large-scale retrieval applications."
"The Puzzle library is designed to quickly find visually similar images (GIF, PNG, JPG), even if they have been resized, recompressed, recolored or slightly modified.
The library is free, lightweight yet very fast, configurable, easy to use and
it has been designed with security in mind."
"Windsurf is a general framework for efficiently processing content-based image queries. With particular emphasis to the region-based paradigm, Windsurf provides an environment where different alternatives of the paradigm can be implemented. This allows such implementations to be compared on a fair basis, from the points of view of both effectiveness and efficiency."
"pHash is an open source software library released under the GPLv3 license that implements several perceptual hashing algorithms, and provides a C-like API to use those functions in your own programs. pHash itself is written in C++."