From the webpage:
LIRE (Lucene Image Retrieval) is an open source library for content based image retrieval, which means you can search for images that look similar. Besides providing multiple common and state of the art retrieval mechanisms LIRE allows for easy use on multiple platforms. LIRE is actively used for research, teaching and commercial applications. Due to its modular nature it can be used on process level (e.g. index images and search) as well as on image feature level. Developers and researchers can easily extend and modify Lire to adapt it to their needs.
The developer wiki & blog are currently hosted on http://www.semanticmetadata.net
An online demo can be found at http://demo-itec.uni-klu.ac.at/liredemo/
Lire will be useful if you start collecting images of surveillance cameras or cars going into or out of known alphabet agency parking lots.