Yahoo! Buzz was opened to public just a few days ago and a lot of hacks and plugins are coming pretty soon. For WordPress Travis Johnson created the WPBuzz plugin. Installing is so simply as drinking a glass of water. You basically download the zip file containing the plugin, upload it to your plugins folder, and activate it the next time you access your plugins page. You can also change the way the button looks by selecting the type of button you want in the settings page.
You can find also Buzz Buttons that with some modifications of your template can be integrated to any type of blog.