WP Captcha-Free is a lightweight plugin that blocks automated comment spam without using captcha (image verfication).
I use this on every website I build, professional or personal. Fantastic and really cuts down on spam. Have been using this since my first WP site in 2009 and would recommend it to everyone!
Upgraded to 3.6 and then it started blocking anyone who was not registered from commenting. My first thought that the failure was due to another plugin that I recently installed, but deactivating that had no effect. I was still getting ENTER YOUR NAME AND EMAIL and ENTER A COMMENT ERRORS no matter what I did or what settings were in place. When I deactivated this plugin, my troubles magically disappeared and all went back to normal. Didn't need it anyway as I have an anti-spam plugin that works very well in blocking spammers.
Used to get 1-2 spam postings per day. Haven't gotten a single one since I installed WP Captcha-Free. I may be targeted by mostly unsophisticated spammers, but this plugin certainly curbed my spam infestation. Super easy install. Highly recommended. Many thanks to the author.
I stumbled on this a while back and installed it in two WordPress sites I use. It keeps out almost all spammers, and without any captchas to worry about. I love it!
Since I installed it, I hardly receive any spam any more. And the little rest that comes through, is filtered by Akismet.
However, since there is only once in a while something in the Spam folder, it is easy now to actually read these "comments" and make sure, that no real comment gets deleted accidentally.
You want a spam filter to knock out all the spam but never stop a genuine comment. WP Captcha-Free has knocked out all the spam I was getting, and when I've tested it several times myself or via a friend, the comments were accepted no worries. So it seems to be working perfectly, but of course I can't know for sure if any genuine comments are being stopped.
It is quite invisible to the user, the only thing you notice is a slight delay in a comment being accepted.
I think it's brilliant.
Update: 1 year later, I'm still immensely happy with this plugin!!!