Here are some good pvp addons I use. If anyone has any other good pvp addons, please post them, I am always on the look out for new ones.
Afflicted - It basically keeps track of enemy play cooldowns, spells, and buffs. I have seen a lot of other addons do this, like NECB, but a nice feature of Afflicted is that you can customize it say in party, raid, or raid warning when CDs are about to be ready. For example, I have mine set to say "PSYCHIC SCREAM READY IN 5 SECS" in the raid warning channel when I am doing 2v2 arenas.
WoWInterface Downloads : PvP/Arena/BattleGrounds : Afflicted2
Proximo - An arena addon that puts the other teams players in a box showing their class/name/health and all that on mouse over. If other members of your group have Proximo, it will update your screen on their mouse overs as well
Proximo | World of Warcraft Addons | World of Warcraft @ Curse.com
Capping - It uses Ace (the back bone for a ton of great addons
Main Page - WowAce Wiki ) to keep track of BGs. Lets you know how long until a win in AB and EOTS, puts the flag carriers name (clickable) next to the flag icon in WSG and EOTS, and also show how long until a node/base will cap amongst other things. Also, shows on the mini map color coded icons for players with their party number.
Capping - WowAce Wiki Natur EnemyCast Bar - Keeps track of debuffs/buffs, cast, power gains, and (important to me) diminishing return timers. Similar to Afflicted but more robust and useful outside of pvp as well.
Natur EnemyCastBar | World of Warcraft Addons | World of Warcraft @ Curse.com