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