What desktop environments can group window icons in the taskbar like Windows 7?

Unity's launcher groups windows belonging to one application in the same icon.

For example, in this screenshot, I have two Firefox windows open, and one Thunderbird window open. The Firefox icon in the launcher has two corresponding arrows on the left to indicate this, and the Thunderbird icon has one arrow. If I click the Firefox icon, I can choose between its two windows, which are immediately rearranged and laid out on the screen.

• LXDE's LX Panel has this capability. Right-click the taskbar and select "Task Bar (Window List)" Settings. Check "Combine multiple application windows into a single button". If "icons only" is also selected, there's no visual indication that an icon may represent more than one window.
• There's also XFCE4-panel in which a task list item could be added [1].

In Xfce, you can easily configure the Window Buttons plug-in on the panel to emulate a Windows 7-like look & feel. See screenshot:

To do so, right-click on the plug-in's handle, access Properties, then uncheck the Show button labels option. You may also consider modifying the Sorting order option to something that you prefer (personally I always go for None, allow DnD).

Another possibility in Xfce is the less well-known xfce4-tasklist-plugin, which specifically "mimicks the Windows 7 taskbar (pinning applications, etc)".

See this related discussion on xfce-dev ML: tweaking Window Buttons defaults.