TinEye - Reverse Image Search to help find an image's source, duplicates or altered versions.
NISE - The NISE Image Search Engine is a reverse image search engine (like tineye.
GImageReader - gImageReader is a simple Gtk/Qt front-end to the Tesseract OCR Engine.
IQDB - Multi-service image search
Yandex.Images - search for images on the internet, search by image
isk-daemon - isk-daemon is an open source database server capable of adding content-based (visual) image...