Spaces is pretty powerful. There is a similar type of function in Ubuntu as well, but you can have different windows from different apps open and fly between open windows and apps- much more powerful than the dock system in windows (xp). You could for example, if you were managing websites, have an open space for each site, photoshop open in one, illustrator in another- keep everything neat and tidy. Shortcuts can also be added in each window, allowing files to be moved around very fast. Also ther are spring loaded windows- something I really miss in XP- no need for copy and paste (although that is there too).
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