These are some approaches i can think of instead of a text based captcha.
The image idea where users are asked to spot the odd one out like demonstrated or find all the similar images like mentioned in here.
Also a picture with a part chipped in could be shown and chipped pictures could be given as options
like find the missing part from a jigsaw puzzle.
The image which would be shown is http://imgur.com/uefeb08
http://imgur.com/KEJqCg3 is the picture which would be the correct option.
The other options could be rotated versions of this , which would not be so easy for the bot to match. (unless it somehow worked some digital processing algorithm and matched the color gradients or something like that).
This is a good option for people who do not know english or are illiterate and maybe would not understand questions like : is this a bird , plane , superman? after being shown a picture.
Tell me what you think
(Sorry to upload those images on imgur. i dont know how to put them on the wiki .Hope that is ok)