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 moderator
Links are highlighted in the comment body, to reveal hidden or misleading links
If your web host is unable to reach Akismet’s servers, the plugin will automatically retry when your connection is back up
Moderators can see the number of approved comments for each user
Spam 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.