Akismet. Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog's "Comments" admin screen.
Major new features in Akismet 2.5 include: A comment status history, so you can easily see which comments were caught or cleared by Akismet, and which were spammed or unspammed by a moderatorLinks are highlighted in the comment body, to reveal hidden or misleading linksIf your web host is unable to reach Akismet's servers, the plugin will automatically retry when your connection is back upModerators can see the number of approved comments for each userSpam and Unspam reports now include more information, to help improve accuracy PS: You'll need an Akismet.com API key to use it.
Keys are free for personal blogs, with paid subscriptions available for businesses and commercial sites. Jetpack by WordPress.com. The most popular WordPress plugin for just about everything.
Security, performance, marketing, and design tools — Jetpack is made by the WordPress experts to make WP sites safer and faster, and help you grow your traffic. We guard your site so you can run your site or business. IntenseDebate Comments.