Like Yahoo! Mail and Hotmail, Microsoft Outlook uses the concept of folders to organize email messages - the problem with the folder approach is that an email message, task or contact can be stored only in one folder at a time unlike Gmail where you can place an email into any number of virtual folders (aka Labels).
Related guide: Most Useful Microsoft Outlook plug-ins
If you are missing tags in Outlook, get Taglocity. It's a Microsoft Outlook add-in that adds tagging feature to your Outlook interface. You can assign multiple tags to your emails messages and quickly filter content in folders based on tag names.
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 is available for Outlook 2003 and Outlook 2007 on Windows XP and Windows Vista. There is a free version of Taglocity for Personal Use though it imposes a limit on the number of tags.