What are some alternatives?
When comparing Search by Image Extension and dooblet, you can also consider the following products
TinEye - Reverse Image Search to help find an image's source, duplicates or altered versions.
RevIMG - RevIMG is a reverse visual search engine.
VisualData - Instagram for Visual Datasets
NooBox - NooBox is a Chrome Extension that provides useful functions, such as: Multi-engine Reverse image search, Extract Images, Screenshot & Search
GazoPa - 27 Oct 2009 . Gazopa, a search engine that uses features from an image to retrieve similar images, has been in private beta since it launched during TechCrunch50 last year.
Bing Image Search - Now w/ related searches, Pinterest boards, and where to buy