DéjàVu is an interface AddOn for World Of Warcraft. DéjàVu keeps track of all the players you have targeted while playing the game. You can list each time you saw a particular player, as well as information about the player's guild at this time, his level, your level, and if you were in a party or not.
 
Simply type "/dejavu" or "/dv" to open the DéjàVu window in game. Disable all chat messages with "/dv silent".