Pretty basic. Cleans up the enemy Arena frames and makes them more customisable. You can add mouse clicks to it so pressing right click on an enemies frame will cast a CC on them etc.
An audio addon, this calls out enemy CDs used. You can set up exactly which spells you want 'said'. It's good for new people or people who struggle to keep up with Arena, as it takes a lot off what you normally have to keep track of. (Don't make any PvP videos with this turned on though!)
Removes the "Leave Queue" button from Arena queue pops. Saves you any embarrassing "ah crap, i pressed leave queue by accident" moments.
Juked or InterruptBar
Not important for melees, very important for casters and invaluable for healers. Tracks interrupts used and lets you fake easier. I'd say Juked is probably better and easier to set up than InterruptBar, but I haven't tried IB since it got updated recently. Juked can also track all enemy CDs, so you can see when their next Deep Freeze is for instance.
Adds icons to portraits so you can see CC more visually. Every little helps.
Adds a number to the CD of your spells, makes it easier to communicate CDs with partners and know when you've got stuff coming up.
Amazing for RBGs and helpful for Random BGs. Helps with targetting people for swaps and seeing who is in range and what health their team is on.