1. The First and one of the most important is Gladius.
Gladius is an addon which replaces blizzard's default arena unit frames for your enemies. It can also be configured to announce the use of important cooldowns such as PVP trinkets, as well as showing timers on crowd control on your opponents. Gladius for many arena players is a must have, and it certainly makes targeting and keeping tabs on your opponents much easier.
2. The second addon is one which I find incredibly useful, Power Auras.
Power auras is incredibly configurable, it allows you to set up screen graphics and sounds to relay information to you. Many people use this for things such as procs, indicators of low health, cooldowns, and other vital information. For example, I have mine set up to flash a snow flake on my screen when my target is frozen/in any situation in which I will gain the benefit of shatter. There are many other addons which will do some of what power auras does, but not many which will do everything it does.
3. Get yourself a cast bar addon! I recommend Quartz.
Now I don't know about you all but I used to think that I was fine with the default cast bar, and then I started moving everything around and a friend turned me on to quartz. Quartz has the ability to let you take latency into account by showing the effect latency will have on your casts. This allows you to get chain casts and get that shatter combo which gets you that kill.
4. The final addon which I recommend for arena play is a good unit frames addon. My choice is shadowed unit frames.
Now I have talked in the past about unit frames, but after experimenting, my current choice is SUF because of it's apparent simplicity, but huge amount of options. Now I have talked in the past about pitbull and all of it's possibilities, but it is a bear to configure to say the least. SUF took me 10 minutes from install to being ready to enter arenas. Now I did do more tweaks along the way, but generally I was happy with it.
Today we looked at what I see as valuable addons for PVP and arenas in particular. One of the most important is gladius, it allows you to get control of your enemies. Power auras shows you a lot of valuable information in a completely customizable way. Cast bar addons such as quartz all you to really step up your game even if it is very small. Finally, a unit frame addon allows you to customize what it is you would like to see, and allows you to keep information at hand. Do you have any addons you would recommend for people who are PVPing?