Most underused/unknown SharePoint feature
You can tag any element on your portal with the comment. The comment becomes visible for everybody who are at the same page and have Discussion tollbar open. It works as instant messaging too.
When you are at any page of your portal go to the IE explorer menu bar Vew->Explorer Bars->Discuss.
Use the button “Insert Discussion in the Document”. The server will highligt all places on the page you could tag with a comment.
Type in your comment.
That feature is available because Office Server Extentions are part of Sharepoint installation.
Link about Office Server Extentions and thier relation to SharePoint.