While much of this guide may seem like common knowledge for most, I am putting it out here to possibly give helpful information to someone who is newer to the game and might be looking for addons that will help in arena.
Gladius:
http://wow.curse.com/downloads/wow-addons/details/gladius.aspx
Why is Gladius so important? Gladius, much like the default blizzard enemy arena frames is there to make it easier to see who your opponent is, along with allowing you to click on them. What makes gladius much better is its ability to be customized to fit many different shapes and sizes, along with allowing you to even cast spells by clicking on the enemy.
Gladius is a must have mod for anyone who is interested in wanting to be successful in arena.
While Gladius may help with clicking on the enemy, this next addon is very helpful when used in conjunction with Gladius.
Diminishing Returns:
DiminishingReturns - Addons - Curse
This easy to use addon allows you to see the diminishing return on the spells (and type of spell) that each enemy has.
For those who do not know what a diminishing return means:
Certain spells and abilities are less effective against player characters if they are used frequently within a short period of time. When a spell with diminishing returns is used against a target in PvP, the first effect has full duration. The effect then diminishes by 50%, then 75%, then the target becomes immune. Spells must be used on the same target within 15 seconds of the *end* of the duration in order to be diminished. In other words, if a target hasn't had a stun effect active on them for more than 15 seconds, the next stun will have full effect."
This allows you and your arena partner to coordinate crowd control effects on the enemy team, giving yourselves a better chance at landing a kill, or surviving the enemy team's damage.
Spell Alerter:
http://wow.curse.com/downloads/wow-addons/details/spell-alerter.aspx
This addon's name is very self explanatory at what it accomplishes. It simply announces what the enemies are casting in pvp. This is very helpful because it can help you see whenever the enemy team is making a target swap to you, allowing you to position yourself next to a pillar so you can los (line of sight) the spells. This is also very helpful to healers to know when to los an enemy priest that is trying to mana burn you, or an enemy trying to crowd control you.
Interrupt Bar:
http://wow.curse.com/downloads/wow-addons/details/interruptbar.aspx
This addon is probably the most game changing of all the mods. Many times if you could just fake that one interrupt and know when it will be back up, you could win the game for sure. Thankfully with Interrupt bar, you will be able to track all interrupts and know exactly when to fake cast, allowing yourself to have maximum uptime when it comes to casting spells.
Threat Plates (used with Tidy Plates):
http://wow.curse.com/downloads/wow-addons/details/tidy-plates-threat-plates.aspx
This addon replaces the default enemy (and ally) bars over their head with a much more informal graphic. The threatplates bar makes it easy to tell which enemy is which, and also scales down things like totems which the default enemy plates usually makes large.
While this simple addon guide may not help the majority of people, I hope that I have been able to help at least one person.