Reverse image search refers to the search type where you don’t need keywords at all; instead you just need to provide a base image for the search engine to find more images like that one.
This is not a very popular type of search engines because it is new and not really well-known.
|Reverse image search tool||The algorithm (seems to be) based on|
|Gazopa||Color and shape similarity|
TinEye a reverse image search that also has a handy FireFox addon is best for searching exactly matching images: those that look identical to the base one but are available in different resolution and from different sources.
It can be a great tool for finding stolen and / or un-attributed images:
Byo image search seems to be based on color palette similarity primarily but does include theme based algorithm:
Gazopa (quite a new tool that launched last month) allows to upload a base image from your computer or even draw one yourself. The similarity is based on color and shape match which works amazingly great but can still be sometimes quite confusing. Look how a broccoli is perceived as a tree:
Can you think of any cases when those tools can be particularly useful?