We all know that those security codes don't work as well as we want them to. You know where they ask you to type in the letters you see in a distorted image.
WordPress currently has nothing like that. But I suggest that we implement something similar to it.
We ask a question "What color is grass? (answer in lower case)"
Any human can easily answer "green" but it is a lot harder for spam bots. I've recently seen these types of things pop up in a couple of places and i seems to work very well.