How do I put category name as a header on single posts?

What I am trying to achieve is to add the category to the top of each single post like “posttitle” does on the top of each category loop when you click on the category in the sidebar.

My site is http://www.samsenovich.com. You will notice, when you click on a category in the sidebar, a category header is added to the top of the main window. This is what I want in my single posts.

I have smart navigation plugin activated, so once you are in a category, and once you click on an entry, you still remain in that category (with navigation to “previous” and “next” entries strictly in that category).

The problem is that I want users to know if they are in a category.