This was supposed to ship with IE7, but Firefox's recent market gains forced Bill's hand. Microsoft adds tabbed browsing to Internet Explorer today, via the latest MSN Search Toolbar.
Bundling branded search with browser features in an attempt to beat Google is typical Microsoft sneakiness, reminiscent of the days when they bundled IE with Windows to kill Netscape.
The new toolbar's implementation of tabbed browsing is clunky, at best. Opening links in new tabs takes two clicks, opening bookmarks in new tabs takes three, and links made to open in new windows can't be made to open in new tabs. Once Microsoft finally figures out tabbed browsing, maybe they could work on getting IE to follow web standards, detect RSS, and do all the other cool stuff other browsers are already doing.