Akismet protects your blog’s commenting section from hyperlink spam and spammy anchor text URLs posted by users as well as spam bots. The plugin also protects your installation from TrackBack and Pingback spam (which is also known as sping).
Wordfence is a powerful WordPress plugin which protects your website from Distributed Denial of Service (DDoS) attacks, malicious web crawlers and other types of attack techniques commonly used by hackers, script kiddies etc. The plugin can also scan your entire installation in real time for various types of existing as well as new vulnerabilities like Heartbleed etc.
CloudFlare offers a free security plugin for your self-hosted installation of WordPress, which make use of spam database hosted and maintained by Project Honey Pot to protect your blog from hackers, attackers and malicious IP addresses.