http://wordpress.org/extend/plugins/wp-captcha-free/
I've been using this for nearly 2 years, and I've had about 3 spam messages in that time. Before this, and with Akismet installed and running, I was getting about 12. [b][i]per day[/i][/b].

I don't like captcha, and I don't want my legitimate users to experience it. Can this be made into a plugin for WP? It's just

You mean a plugin for ZP, it's already a WP plugin...;-) Btw, we have already a captcha function (did not look at that one actually yet).

Given its description, I am sure it can be made a Zenphoto plugin. Not on our list, though. Also given its description I would think that any respectible hacker who wished to crack it couled easily do so.