Captcha is an essential part of a lot of websites to protect us from spam. However you have to have some degree of usability on your site.

I went to Yahoo to retrieve a forgotten password and was prompted with the following 4 captcha's in a row. The icing on the cake is the last screenshot after I finally got one that worked on the 5th attempt.

zeSs, z3Ss or zbSs?

04ILd or 84ILd or 041Ld

dzfo8..... I frigging give up 

Aeon?? Is this even friggin English?

Oh the joy, now I have to go back and start again.

Yahoo I suggest you take Microsoft's offer