The best link (URL) shortening extension for Safari 5. It supports a dozen different shortening services. And it allows you to tweet your short links directly to Twitter from right inside Safari 5.
You can click the toolbar button to shorten the link of the current page.
If you hold down the [option] key while clicking the toolbar button you can short the link and tweet that short link (as well as any other text you want to add) right to Twitter. No copy/paste necessary.
Shortenz Linkz also adds contextual menus to help you shorten links in the current page. So if you right click on any link in a web page you can shorten that link or even shorten and tweet that link.
No other shortening extension has more features or supports more services.