Look for sites with sign in forms that look like this:
Enter your OpenID – that™s the URL of your weblog:
Click the button, and you will be redirected to WordPress.com. If you are already logged in, you will be asked if you want to pass your identity to the site:
Click Yes; just this time, or Yes; always if you don™t want to be asked this question again for this site.
You will be redirected back to the site and logged in, all without using a username or password!
How is that for an intelligent service. However, as Anne Zelenka on Web Worker Daily points out, ideally you may want to map a unique url to your WordPress.com account (take a look at your Dashboard options). Anne also shows you how to use your url as your OpenID credentials even if you don’t point it to your WordPress.com site.