Microsoft has included a new feature, the Attribute Editor, in Windows Server 2008 which allows you to view and modify attributes through two of the native Active Directory snap-ins (Active Directory Users and Computers and Active Directory Sites and Services). This is especially valuable when you need to view and/or modify attributes that are not part of the base schema, such as custom attributes. In the Windows 2000 Server and Windows Server 2003 versions of Active Directory, these attributes could only be modified programmatically or by using the ADSI Edit console. However, in Windows Server 2008, you can now modify custom attributes by using the native tools.
The Attribute Editor is visible by default in the Active Directory Sites and Services snap-in.
To open the Attribute Editor, open the properties page for any object in Active Directory Sites and Services and click on the Attribute Editor tab.
NOTE: The Attribute Editor not visible by default in the Active Directory Users and Computers console when you select Advanced Features from the View menu.
Once the Advanced Features view is enabled, the Attribute Editor will appear as an additional tab on every object you open in the Active Directory Users and Computers console. To edit an attribute, click on the attribute you want to edit on the and then click Edit.
In the image below, a custom attribute is visible by using the Attribute Editor. This same custom attribute can be modified by using the Attribute Editor as well.
The Attribute Editor also recognizes attributes that are system owned, which cannot be modified. For system owned attributes, you can only use the View button to view the value of the attribute. 
John says:
Let’s say I want to add an attribute “homeMail” that applies to all users (new or existing ones).
How can I accomplish this?
John Policelli says:
You need to create the attribute in your forest, which is also called extending the Active Directory schema.
You can find all of the necessary details in the following paper from MS (which I wrote): http://johnpolicelli.wordpress.com/2008/08/21/structured-active-directory-schema-management-at-microsoft/
John says:
I have made many attempts but no avail.
Could you rather forward me to an tutorial?
Many thanks,
John
John Policelli says:
See the following:
http://technet.microsoft.com/en-us/library/cc759633.aspx
Chuck says:
This is one way to edit attributes in AD; but its really painfull. I have a free tool that I developed that makes it real easy and lets you perform bulk changes. Theres even a video that shows how to use, it also for free.
http://www.arconi.com/utilities/active-directory-utils/activedirectoryeditor.html
Mike says:
Is the attribute editor only available if the forest is at 2008 functional level? I tried your steps in ADUC on Windows 7 and from ADUC on a 2008 server, but did not see the attribute editor tab when pointed at a 2003 domain.
John Policelli says:
See the following:
http://social.technet.microsoft.com/forums/en-US/winserverManagement/thread/6e6ef6bd-b5c9-4f16-b346-097832e3b93c/
Graham Knight says:
Is it possible to reach the data in the 2008 attribute editor from VB or VBA?
John Policelli says:
The Attribute Editor tab simply exposes the attributes in the GUI. There are several programming interfaces that can be used with Active Directory, such as ADSI, MAPI, and LDAP. Here’s a link with some more information: http://technet.microsoft.com/en-us/library/cc785712(WS.10).aspx
Stuart Clowes says:
The attribute editor is also available in Active Directory Administration Center.
See http://www.stuartclowes.com/2011/10/displaying-hidden-attributes-in-active.html for a screenshot.
In Active Directory Users and Computers, I found that the attribute editor is not displayed if you right-click on a user you locate through a Find operation. Can others reproduce, or am I missing something?
John Policelli says:
As per the note in my post “The Attribute Editor not visible by default in the Active Directory Users and Computers console when you select Advanced Features from the View menu.”