Gladius adds enemy unit frames to arenas for easier targeting and focusing. It is highly configurable and you can disable most features of this addon.
PVP Trinket status
Also announces the following events to party/RW (not turned on by default);
Trinket announcement - announces when the trinket is used/is ready again.
Drinking announecment - announces when an enemy has started to drink.
Enemies on low health - announces enemies that are about to die, you can set the percentage yourself. *
Gladius can be downloaded from http://curse.com
* Bold text because, embarrasingly, I didn't realise Gladius did this until I started watching live feeds from AJ. Maybe others aren't aware either? Hmm...