Fighting forum spam?

Rubbish. Some of automated programs just stop/pause and wait for a human to come along and answer it.

 

If you dont get it, leave it.
Neither you nor I have to prove anything.

 

Then you should look here: http://decaptcher.com/client/

Using reCAPTCHA they will have a hard time circumventing captcha images.

 

reCaptcha's been known to be readable for quite some time now.

 

Yes, I noted your remark the first time. Nonetheless, adding reCAPTCHA to my forums reduced spam by 90%. This is empirical fact irrespective of whether you believe reCAPTCHA is reliably readable.

 

Thanks for all the suggestions, I will experiment with them and see what's most effective.