PVP Addons for common BGers or Arena enthusiasts.
Common UI addons (for common buffs or frames)
Pitbull - standard Unit Frames addon
Xperl - basically another version of pitbull
Gladius - It's an arena addon with unit frames for the enemy team, showing their castbars, buffs, CC, debuffs, etc. It also has a handy function of alerting you on enemy trinkets and such.
Bartender - customizable action bars
Bindpad - takes away need for most of your actionbars, allowing you to bind your abilities outside of bars.
Buffbars, Castbars and other:
Quartz - Castbars for everything, along with a buff function.
Elkano's BuffBars - buffbars for everything. Imo bars can get annoying, but here they are.
SBF (Satrina Buff Frame) - customizable and redone blizzard buff bars with alert options and stuff like that.
Classtimer - buff bars for player, pet, target, and focus that show CC on them, or special buffs gained (such as Hex on your focus or target, or Elemental Devastation for Enh shamans).
SCT (ScrollingCombatText) - A more customizable combat text addon.
SCT: Damage - Makes SCT show damage you do (in addition to the stuff the main SCT does).
WinterTime - a timer and autoinvite mod for WG.
Afflicted 3 - An enemy spell timer that tracks useful information such as interrupt CD's.
DR Tracker - tracks DR's.
CC Tracker - Tracks any form of CC.
Hopefully these are helpful, and i plan to add more in the future.