The newest version of the MSN Search Toolbar adds tabbed browsing to Internet Explorer. You can check out a screenshot if you're curious to see what its like. Denise from the MSN Search Toolbar team blogged about it in her post Tabbed browsing is here! where she wrote

Tabbed browsing not only makes online shopping easier (and my wallet more broke), it also drastically improves my web search experience! Have you ever gotten annoyed with hitting the back button repeatedly to get back to the original search results?  MSN Search Toolbar gives you the option to open MSN Search results in new background tabs. This is a HUGE time saver because it lets you keep the search results on one tab while links that you click open in the background.

Best of all, Tabbed Browsing with MSN Search Toolbar works in Internet Explorer 5.01 or above! IE7 will also offer tabs but in the meantime this is a great option.

I think it's way cool that the MSN Search Toolbar team has added this functionality to IE. First Trixie and Turnabout, and now tabbed browsing added via the MSN Search Toolbar. It definitely goes to show exactly how extensible IE is as a platform. Of course, I still am interested in seeing what Dean and his merry band have planned for IE7.