It occured to me that when you sign up for a yahoo account, they display a word in jpg that has been twisted, or color-graded, or with lines running hither and yon that 'prove' the user is a person, since a computer can't read the text in the jpg.
Is there any plug-in for doing this in comments? that's the sytem i'd like to see: 'type the word in the picture to post'
just an idea, no doubt someone has already started to run with.