In WoW, Busy Mode can be set by typing
/busy. The name of the character will show up as <Busy>Name, and an auto-response will be sent to anyone who sends the flagged user a tell. Additional information can also be set; for example
/dnd killing an elite. (See Away for more details.)
Patch changes and hotfixes
Patch 3.3.5 (2010-06-22): The <AFK> tag was replaced with <Away>, and the <DND> tag was replaced with <Busy>.