This plugin by Daniel Frużyński enables blog owners to insert some extra content above and/or below your posts, post lists and others. You can do this by editing template files directly, but this approach is a bit inconvenient – you have to edit files every time you want to change it. Additionally this may be not acceptable if you share template between multiple blogs (e.g. while using WPMU).
The Custom Headers and Footers plugin does not modify post contents on the fly – it uses WordPress loop directly to insert header when loop starts, and footer when it ends. With this approach you will avoid unexpected side effects, e.g. displaying header only (or part of it) as a post excerpt.
You can also add extra code tosection of blog (e.g. additional meta headers), and to blog footer.
Plugin homepage: http://www.poradnik-webmastera.com/projekty/custom_headers_and_footers/#en