isk-daemon - isk-daemon is an open source database server capable of adding content-based (visual) image...
Google Images - Google Images is a search service owned by Google that allows users to search the World Wide Web for image content.
IQDB - Multi-service image search
SauceNAO - SauceNAO is a reverse image search engine.
TinEye - Reverse Image Search to help find an image's source, duplicates or altered versions.
PicsLikeThat - This is a visual and semantic image search system finding images on Fotolia.