Where can I get code like you have at registration for keeping out spam spiders?

[cdaniel]

When I registered, there was an image generated and I had to enter the letters and numbers to prove I was a live person. Where can I get code like this. I have numerous web sites and every day they get junk responses.

I want something I can use for any web site, not just for my vBulletin forums.

Thanks,
Cynthia