http://tinyurl.com/2pb7f8TinyUrl lets you shrink any long URL in your address bar or link on the page to something you can easily email or Twitter with a single click.
NEW: Now any link on a page that has a tinyurl.com address will contain a tooltip to tell you what the full URL for it is. Just hover your mouse over the link for a moment and you'll see the full URL appear as a tooltip.
The TinyUrl Creator is accessible in the Tools menu and in the right-click context menu. You can even add it to your toolbars as an icon (View > Toolbars > Customize).