Tactile cues
When I accidentally bought the same toothbrush as my flatmate, I decided I needed to do something to my brush to let me know which one was mine and avoid the confusion (and horror) of using the wrong brush. I wrapped a piece of electrical tape around the handle, this acts both as a visual cue to differentiate between the two, but also as a tactile cue so that when one picks up the brush, it feels different in the hands - even if they are not looking at it.
This kind of tactile cue could be used with many other everyday objects to differentiate without the use of visual information. Bumps and textures could be used navigate a product or set of products - particularly useful for blind user groups, or applications where the eyes are otherwise occupied, for instance whilst driving.