From Guy Kawasaki. All of these are so true, especially this bit used by Yahoo quite extensively (it's now gotten to the point that if you want to send another yahoo user an email, you have to type in the confirmation codes.

Unreadable confirmation codes. Don’t get me wrong: I don’t support spam or robots creating accounts. A visual confirmation graphic system is a good thing, but many are too difficult to read. For example, this is what I got when trying to create a Yahoo! account. Is that an uppercase “X”? Is the last character an “s,” “5,” or “S”? Maybe this only affects old people like me, but it seems that all one merely has to prove is that you’re not a robot so a little bit of fuzziness should be good enough. For example, if the code is “ghj1lK” and someone who enters “ghj11K” is close enough.