The "carbon copy" (cc) feature of Outlook isn't particularly useful. You can copy multiple people on an e-mail you send to someone, but that's the extent of it. If you're looking for an exceedingly powerful way to use a carbon copy e-mail feature, you'll want to give Visible Carbon Copy ($20, 10-day free trial) a whirl. It lets you send hidden, personal notes to individuals on a cc list, and even send attachments to some people on the list and not others.
VCC runs as an add-in to Outlook 2007, and appears as a new menu item at the top of Outlook. When you create a new email, click Vcc at the top of your screen, and you get access to the program's features. From there, you can create messages that only certain people on a cc list can see, as well as attach files that only they can see.
At first, Visible Carbon Copy may be a bit confusing to use, because you need to create a "group" of people who will be able to see attachments and notes that other people to whom you're sending the email won't be able to see. If you only want one person to see the notes and attachments, you can create a group of one person. Particularly useful is that you can preview your messages before sending, so that you will be able to check who will see the hidden notes and attachments, and who won't.
Most people will likely not need this program because they rarely, if ever, need such sophisticated use of carbon copies in email. But those who need it will find Visible Carbon Copy a keeper.
Note that when you install this program, it installs a number of necessary Microsoft-related tools on your system if you don't have them already, such as Microsoft Windows Installer and Microsoft Office Interop Assemblies.