If you have WordPress blog and ever wanted to create a new one, you had to register a new account. It is No longer! required and you can create unlimited Worpdress.com blogs on a single login.

Matt has written about this feature.

Now when you go to wordpress.com/signup as a logged in user, it lets you simply add another blog to your stable. As many as you want, need, or desire — no limits. You may also notice that when you're logged in the WordPress.com homepage gives you a link to the admin section of all blogs that you have access to.
You can now manage all the blogs you want with a single login and user for all of them. Enjoy

Update: If you have multiple accounts, It is recommend going to Users > Authors & Users > Add User From Community to make one of them an admin on both blogs.