Metal Captcha
// edit: sense of humour failure. I care too much about blocking spam :)<p>Sorry, but this is harder for humans than bots.<p>The logos are quite distinct and not randomly deformed, so it's easy for even a simple image recognition algorithms to tell them apart.<p>And the set is very small (for this problem anything smaller than millions is very small), so it's feasible to manually tag enough images for bots to brute force it.