The firefox addon
https://addons.mozilla.org/en-US/firefox/addon/firecoin/ lets users easily donate BitCoins to site owners. It gets your BitCoin address from a metatag named "bitcoin". It also allows the site owners to provide a message and a URL to a thanks page.
This plugin makes it easy for you to add these tags to your headers.
Please note that this plugin will promote the Firefox addon in a HTML comment in the header, which is only visible if a user would inspect the HTML code. It wont show the link anywhere else.