For IE: Tabbed Browsing.
For Firefox: Bookmarks, Mail Alerts, and Anti Spy.
And the big change:
Speaking of changes, you must have heard about del.icio.us by now! People use del.icio.us ito save and share web favorites. We’ve just created a del.icio.us button for our US users. You add it to your IE or Firefox toolbar to get easy access to your del.icio.us account from anywhere on the web. If you already have toolbar installed, you can add it with a click, otherwise, find it on the Add/Edit Buttons page in the “Personal Tools” section.