Gmail uses Labels to separate your mails.If you want to use the same tag in outlook?How can you do it?Here is the solution.
Taglocity. It's a Microsoft Outlook add-in that adds tagging feature to your Outlook interface.Similar to del.icio.us, you can create a tag cloud of your email messages where the varying font sizes help you visualize the number of messages under that tag.
Taglocity available for 2003 and 2007 also.