Please Note: This add-on will not function without ElvUI installed. You can download it at http://www.tukui.org, don't use the Curse version, it is outdated.

For more information see this (Wiki).

Automation:

Auto release in battleground after player dies.
Automatic changing of the watched faction on your reputation bar to the faction you got reputation points for.
Automatic role detection for battleground and party/raid groups.
Automatic selection of highest vendor value quest rewards (you can still pick another if you like).

Data Texts:

Fully configurable extra datatext panels attached to the bottom of the three main actionbars.
Target Range data text module displaying the range to your currently selected target.
Item Level data text for a quick glance at your currently equipped item levels.
Quick datatext configuration by CTRL-ALT-RightClick on any datatext panel.

Equipment:

Equipment Manager with automatic changing of equipment set when changing specialization or entering a battleground / arena.
Equipment Set Overlay for your bag and bank slots showing the equipment sets for an item.

Farming

Extra action bars for farmers while visiting Sunsong Ranch showing seeds, farm tools and portals.
Automatic target and planting of seeds when in range of a tilled soil (quick planting of seeds).
Automatic drop of farming tools when leaving the farming area (configurable).

Mini map

Location information above minimap with coordinates.
Mini map buttons can be skinned and placed on their own bar that can be moved around.
Hide mini map during combat (disabled by default).

Nameplates

Threat percentage on nameplates for target, mouseover and boss nameplates.
Target by count on nameplate for the number of party / raid members targetting that unit.

Raiding

Raid Marker Bar for quickly placing target and world markers.

Tool tip

Player Raid Progression information on mouse-over tool tip (requires holding 'shift' key)

Unit Frames

GPS Locator for target and focus frame, showing direction and range to party or raid member.
Heal Glow: Direct AoE heals will let the unit frames of the affected party / raid members glow for the defined time period.
Hide role icons (healer/tank/damage) on unit frames while in combat.
Class Icons for the target frame.

Visualizations

Adjustable transparency for the ElvUI interface movers.
Attack icon on target frame when target is not tapped by you or your group, but you still get credit for the kill when attacking.
Eclipse prediction on eclipse bar for Balance Druids.
Improved visibility for disconnected party or raid member (lightning icon in frame).
Improved visibility of druid eclipse state indicator.
Item levels and durability information on the character screen.
Watchframe visibility configuration, based on city (resting), pvp, arena, party or raid.

Advanced Features

Extended UF Tag framework, for more information see this Wiki.

ElvUI Enhanced