Several patches ago, a new hidden chat channel was introduced, for the purposes of communication between players running the same addon. Deadly Boss Mods and Omen are two popular examples of addons which use this channel.
Snoop prints out all the messages sent over the hidden addons channel. It is intended to help addon authors debug their communication; it's also useful if you suspect that an addon is flooding you with traffic but don't know which addon it might be (this should be less of an issue after 4.1, which only sends addon traffic to clients which register for it).
Snoop usage (typing "/snoop" by itself prints this):
/snoop N -- Sets output to go to chat window N.
/snoop print -- Prints each chat window's number in that window for reference.
/snoop on|off -- Begins|ends copying addon channel text to the selected chat window.
/snoop esc -- Toggles the formatting of control chars into escape sequences (default on)
/snoop pipe -- Toggles the formatting of raw '|' chars into visible '|' (default off)
Here is an example of a line copied from a CT_RaidAssist message (this happens to be the message sent when the player "Spi" is set as MT #5):
<CTRA>(Farmbuyer->RAID): SET 5 Spi