Safari 5 brings whole new extension framework, allow 3rd-party HTML/CSS/Javascript based extensions to run as toolbar item, in-page code, etc.

Here are my extension(s), some for testing, others may useful for more people. URL Shortener

This is a toolbar button extension. When you browse a page, click on the button and it will bring you a confirm dialog including the page title and the shortened URL which you can copy & paste.