Right click any player in chat to add ChatNotes to that name.
WoW's built-in Guild Notes
ChatNotes will show Guild notes only if there are no Chat Notes, or Friend's Notes stored. If you don't like a specific Guild Note your guild leader has added. Create a Friend note, or ChatNotes to override it.
This shows the Note for 'Stabgasm', being displayed when she types a message into one of the chat channels. The same function works for Wispers, Party, Guild, and Say messages.
WoW's built-in Friends Notes.
ChatNotes will show Friend's notes only if there are no ChatNote Notes stored.