This is very cool, the team at Clarifai have launched an API that allows you to access a very advanced image recognition and tagging service.
Essentially your App or service can present an image to Clarifai who will match it against their image database and recognise it and auto tag it.
I tried it a few times and it seemed to work flawlessly each time.
Clarifai is pushing the limits of practical artificial intelligence. Their image recognition systems held the top 5 spots for classifying objects in images in the recent ImageNet competition and they are claiming their latest systems go even further, achieving lower error rates at 10x the speed with 1/5th of the memory footprint.
If you are developing an app that needs to work with images and you want to present a richer meta data with each image it makes sense to have a service like this auto tag and process it for you rather than having to build the capability yourself.