Google Image Search is a Firefox add-on that makes look-up for similar images even easier by letting you right-click any image, and use the context menu to have Google search similar pictures for you. Google will access information about where on the web the image had been used, and it will also filter out all other similar images.

 

To search an image, just right-click it and hit the Google this image option in the context menu.

Via Freewares&Tutos