Microsoft's implementation of commenting in Active Directory has always amazed me. In Windows Server 2003, everywhere you look you find wizards and tabs and configuration screens with a location for adding comments. It seems like every little setting in Active Directory could tell its own story through its attached comments.
If you've got a large domain run by lots of Windows administrators or if you've incorporated formalized IT processes, in-object commenting is an excellent way to self-document your environment. Attaching a comment to an Organizational Unit at creation helps large domains understand the purpose and ownership of objects throughout their forests. Those comments can contain information about the creator, the authorizing help desk ticket number and even the reason for the configuration.