You can now use your wordpress.com account as OpenID. OpenID is an open, decentralized, free framework for user-centric digital identity. The first piece of the OpenID framework is authentication — how you prove ownership of a URI.
If you go to a website that requires you to have OpenID to leave comments, then you can use your wordpress.com URL as the sign in ID.
You can learn more about how to use your wordpress.com account as OpenID here.