WordPress Registration Anti Robot

I would like to add a feature to my wp 2.7 registration that prevents robots from signing up. Something akin to a box that require you to add some numbers or something along those lines. In addition, I am unsure of where in the php code I would add the snippet.

Is this available in a plugin, or is there another workaround?

Thanks in advance to anyone who takes time out of their day to assist me.