The never ending war
I'm went over lots of CAPTCHAs including my own and none of them was so successfull as the one i'm using now. recaptcha.net, its free, its for a good cause and so far its not been beatable by a OCR.
You even have it set for this forum: http://www.vbulletin.org/forum/showthread.php?t=151824
Of course sometimes spammers do get over this protection by using real people to post spam.