As of November 2011, API keys are no longer distributed with new WordPress.com accounts. The only service that currently uses API Keys for authentication is the Akismet anti-spam service. For WordPress.com Stats for your self-hosted WordPress installation, please use the Jetpack plugin.
How do I get an API key?
To obtain your API key for use with Akismet, please visit Akismet to select a plan and receive your API key.
What else can an API key be used for?
Older WordPress.com accounts received their API key in their welcome email. As WordPress.com is phasing out use of the API key, they are no longer distributed in that fashion.
Until API keys are permanently retired, you can use your API key to verify your identity if you contact WordPress.com Support to obtain access to your account and you don’t have access to your account’s registered email.
Can I share my API key?
No; please do not share your API key with others. Because they can be used to gain access to your account, you should treat your API key like a password.