I'm searching for a system that will protect my registration page from automated bots that sign up again and again. So far they've been pretty good at evading me, using multiple proxies, different e-mail addresses, and even legitimate-sounding names. Heck they even reply to my activation e-mails!

Anyways, I've seen some sites that have this system where an image is displayed that's got some digits in them. You will need to manually type them in a text box in order to continue. An example can be seen here:
http://addurl.altavista.com/sites/addurl/newurl

I'd like to know if anyone know of such a script out there? I already know about hotscripts but can't find anything there. Thanks!