Adding a Creative Commons mark (marking) allows people to easily see how your work is licensed, and allows search engines to index your page by license.
This works on Wordpress.com as well as self-hosted WordPress installations.
This should now display the CC mark. You may style the blog as you wish. If on WordPress.com, you have to use inline styles, unless you have a paid subscription and can change all the CSS of your theme.
Use this method unless you strongly prefer editing source files or installing plugins and have good reason to do so!
TO-DO: visual guide for this method.
Edit the theme's footer.php or sidebar.php file. Get your license button HTML from http://creativecommons.org/license and paste it into your footer.php (or sidebar.php or wherever you want it to appear).
Use a plugin. This might be the easiest way, but has some potential for problems if the plugin breaks when you upgrade, or it clashes with another plugin. The widget above is usually a better option.
To do this, use the Creative Commons license widget. (That page says it's only tested up to WordPress 2.5, but it seems to be working on our version 2.71 blog without problems. --Chriswaterguy 04:02, 16 May 2009 (UTC)) Note: it's easier to install from the admin interface (click Plugins, then Install) rather than downloading from the link given here.