Since every browser nowadays uses Tabs, people get more and more savvy (if they weren't using tools before that use a tabbed interface) with this user interface methaphor.
I would like to see Tabs coming to the Windows Shell, especially to Windows Explorer, which would make file browsing much better/faster, since you don't have to bother with using the taskbar (i hate it when i have 10 Explorer windows grouped in the taskbar).
To save space, the Tabs could be implemented in the top bar like the contextual tab informations in the Office 2007 Ribbon "Fluent Interface".
It may also make sense to offer an option to switch off Tabs, for conservative users ;-)
A neat option would be an integration with something like Expose on OS X to see all all tabs/windows of one application immediately and one to see all windows + tabs of all applications.