I found today a very good class on phpclasses.org . it can generate animated CAPTCHA images. I didn’t see thing like this idea before and it looks like a good technique to try.
see the examples :

  and  

I see that the first one is so complex to be read by robots but it’s so friendly to be read by humans (color blinds can see this color simply, right?) .

anyway, the class is nice and I’m planning to use it in my coming projects – In Sha’ ALLAH -  after adding more effects and making sure that there is no problems with color blinds .

the class can be found here. it’s the winner of the October 2006 innovation award at phpclasses.org.