Every user of WordPress.com gets a unique API key they can use to access a number of services. This key should be kept as secret as your password — don’t share it with anybody.
To find your key
1) Go to your Dashboard and then click on Users ->Your Profile (or click on your name in the upper right hand corner) and you should see a sentence that says “Your WordPress.com API key is:” followed by a string of 12 letters and numbers.
2) There you have it! You can copy and paste that string to any place where it is asked for by a service you want to use.
3) If your blog is on WordPress.com then you are automatically protected by Akismet.
4) If you have a blog here but you use WordPress somewhere else then use the Akismet plugin to save yourself from spam.
5) You must have registered here at wordpress.com to get an API key. Just having a wordpress blog somewhere else does not get you a key.
To see your API key, login to the dashboard and then use this URL: http://dashboard.wordpress.com/wp-admin/profile.php