We can’t stand spam.
Who can? You have better things to do with your life than deal with the underbelly of the internet. Automattic Kismet (Akismet for short) is a collaborative effort to make comment and trackback spam a non-issue and restore innocence to blogging, so you never have to worry about spam again.
Akismet is free for personal use.
In the spirit of helping the blogosphere as much as possible, we’ve decided to make Akismet free for as many people as possible. We have free API keys available for your personal blog.
Commercial Use
We still need to keep the lights on and the servers humming though. If you are making “mad paper” from your blog (more than $500 a month) or if you’re a Big Company we have commercial API keys available just for you.
How does it work?
When a new comment, trackback, or pingback comes to your site it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down.
Do I have to maintain a blacklist?
Nope! Part of the idea of Akismet is that you’re always protected up-to-the-second from the latest dirty tricks of spammers. There’s no maintenance, no upgrading, no hassle.
Where does the spam go?
When the plugin catches something as spam it saves it in the database for 15 days in case you want to check it out manually and then automatically deletes it. In the unlikely event something gets incorrectly identified as spam you can correct it and it submits the “false positive” back to Akismet for analysis and improvement of our system. If a spam comment happens to get through and you mark it as spam within WordPress, it does the same thing. Akismet becomes more effective the more you use it.
Can I use Akismet for my own blog or site that resides outside WordPress.com but using the API key from WP.com?
Yes! That’s the point. Download the plugin and start relaxing.
Where do I find the API key?
It’s on your profile page, check out this FAQ on WordPress.com.
My API key doesn’t work!
We get this pretty often, try these steps:
* Try the key again, after a few minutes.
* Type in the key instead of copying and pasting it.
* Ensure that your host can make outgoing HTTP requests to akismet.com. (Many hosts block or firewall outgoing connections.)
What if the Akismet web service is down?
One of the reasons we’re doing Akismet is we’ve built up a highly fault-tolerant infrastructure that can handle huge amounts of traffic and processing. However if something ever does go wrong your comments will simply go into the moderation queue.
What do I do if Akismet misses a spam?
When you mark a comment as “spam” in the plugin or submit it using the submit-spam API call, the mistake is noted by Akismet and it learns from your submission.
Help! Akismet is catching a regular comment as spam!
Don’t worry, if you see a regular comment on your Akismet page, just click the “Approve” button or the “Not Spam” checkbox and submit and the comment will be sent back to Akismet as a mistake. The system will learn from your submission, though it may take a day or so in some cases. False positives, as they’re called, are extremely rare and we watch them closely.
How can I support Akismet?
You might consider buying a pro-blogger license for $5/mo. If you would like to participate in the development of the GPL Akismet WordPress plugin or discuss adapting the Akismet API for a different platform, join the Spam Stopper mailing list and introduce yourself and your ideas. The more the merrier. 
Can Akismet be used for other things besides WordPress?
Absolutely. The Akismet API can be adapted for almost any application with submitted content, including forums, wikis, and contact forms. If you want to adapt it for a different system, please join our development community.
What if it shows a different number of comments than it has reported caught?
Some versions of the Akismet for WordPress plugin will hide duplicate comments, making it appear to be a different number caught than displayed.
Should I use any other spam plugins?
We ask that you turn off all other spam plugins as they may reduce the effectiveness of Akismet. Besides, you shouldn’t need them anymore! But if you are investigating alternatives, we recommend checking out Bad Behavior and Spam Karma, both which integrate with Akismet nicely.
Does it work on non-English comments?
Yes, our system does not discriminate against non-English comments. We have happy users from Japan to France.