Honeypot Sep 3rd, 2013 (updated Jul 28th, 2015) Uses https://www.projecthoneypot.org/ to prevent spam. I haven't used it, but it comes pretty highly recommended by others. Use http:BL with Honeypot and possibly Mollom for ultimate spam protection. Read more about Honeypot
Bad Behavior Sep 3rd, 2013 (updated Sep 14th, 2014) A spam prevention tool. Also see Mollom and Honeypot. Read more about Bad Behavior