Arena Mods

Gladius v3

Gladius adds enemy unit frames to arenas for easier targeting and focusing. It shows a health bar, a mana bar below the health bar (optional), a cast bar below the mana bar (optional), a class icon to the left side of the health bar (optional) and important buffs and crowd control durations (optional, overlays the class icon). The frame resizes itself to the current bracket (optional).
Config: /gladius ui
Curse

Battleground Mods

AlarBGHelper

Battleground timer and coordination mod. Features: Auto release; Mini Map tooltip colorizing with class and race (if you have some tracking ability); Resource tracking. Nodes or bases status is shown in a compact frame. Clicking on a base name will: Left-Click: says "Help needed at <base name>", Middle-Click: says "<base name> now safe", Right-Click: says "Massive inc to <base name>"; Shift-Left-Click: says "On my way to <base name>.Hold on".
Curse

BattlegroundTargets

Shows all battleground enemies. Simple 'Enemy Unit Frame' for battlegrounds.
Curse
Review: WoW Insider

Capping

Capping is a battleground timer mod with some extras. Current Features: All battlegrounds/arenas have start timers. Arathi Basin: Node timers/Final score estimation. Alterac Valley: Node timers/Auto quest turnins/Request sync from others with Capping. Eye of the Storm: Flag respawn timer/Flag carrier display/Final score estimation. WSG: Flag respawn timer/Flag carrier display with health percent. Battleground minimap: Rescale/Narrow mode to remove unnecessary map areas and save space/Hide border. Other: Port expiration timers/Queue wait timers/Scoreboard repositioning/Capture bar repositioning.
Config: /capping
WoWInterface

Misc PvP Mods

DiminishingReturns

Diminishing Returns (DR) helps tracking your teammate and own diminishing returns on hostile players by attaching icons to unit frames.
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HealersHaveToDie

Puts a big red cross over opposing players who are healers and rings when you hover or target a unit of the opposite faction who healed someone else during the last 60 seconds (can be configured).
Curse
Review: WoW Insider

Vanas KoS

Vanas KoS is a Kill-on-Sight notifier, that uses every possibility, to detect nearby KoS targets, and notifies the user in different ways. Additionally it records PvP-Stats in outdoor PvP and shows these in the tooltip (configurable).
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Main Mod Sites
UI Forums
Addon Reviews/Info