Are there tools to help labelling images? I need to manually classify thousands of pictures into discrete categories, say, where each picture is to be tagged either A, B, or C.

Edit: I want to do this work myself, not outsource / crowdsource / whatever online collaborative distributed shenanigans. Also, I'm currently not interested in active learning. Finally, I don't need to label features inside the images (eg. Sloth) just file each image as either A, B, or C.

Ideally I need a tool that will show me a picture, wait for me to press a single key (0 to 9 or A to Z), save the classification (filename + chosen character) in a simple CSV file in the same directory as the pictures, and show the next picture. Maybe also showing a progress bar for the entire work and ETA estimation.

Before I go ahead and code it myself, is there anything like this already available?
There are a few tools that you can use to annotate (or label) data. For example, labelme or Labelbox. Have a look at this question for more alternatives.