There was a great "Kill Log" addon in beta until Zenimax messed with the API. Arrgh.
Such an addon which prints "X killed X" while fighting in Cyrodiil would be priceless. There's nothing like coming to really hate your particular enemies by name.
To take it one step further, it would be nice to record such kills (and killed by's) in a book (like in WAR's "Little Black Book"), so I could see a tally of who has my number the most.
Since Zenimax flashes messages in Cyrodiil "You have avenged X's death," isn't there at least some of that data available to addon devs? Sorry, I've no clue what the API is like.