Can you advise me on techniques for enhancing signup protection?

Sploggers here, sploggers there, sploggers, sploggers everywhere.

It appears that since Anti-Splog is young and still learning, it is not yet fast enough to keep up with the twits who want to use my bandwidth in their attempt to scam Google rather than use their brain the way Google wants them to do.

Can anyone suggest a preferred enhancement to my registration system that will cut down even more on these leeches?

Among the things I know of are …

Anti-Splog

Signup Code

Numeric Captcha

Visual Captcha

Is the Signup Code worth considering?

Are there other systems?