You might remember my ranting about the JSSpamBlock plugin of Paul Butler, thinking of porting it to the actual WP release. I have decided to keep an eye on it fixing various bugs for keeping it compatible.
JSSpamBlock extends the comment form with a simple input field for checking if you’re human or not. As this function uses a hook which is also used by the dashboard reply-to-comment function, the latter one throws an error. Assuming that spambots are not authenticated users, I did a workaround on that.
Download here