Akismet filters through the comments on your blog and marks the suspicious-looking ones as spam. When comments are caught, they are sent to the spam section of the comments page in the WordPress admin.
This plugin is the most popular when it comes to spam comment filtering and is free for personal blogs (it even comes pre-installed on all WordPress site). There’s also a subscription for commercial sites. In both cases, you need to enter an API key in order for it to work.
Interested in Akismet?