Leatrix Plus is an efficient and modular addon which offers lots of quality of life improvements to enhance the default user interface.
Click the minimap button or enter /ltp to open Leatrix Plus. Some options have little configuration buttons next to them for further configuration. To report a bug, ask for help or leave feedback, use the Issue Tracker (private messages are ignored). There is a list of slash commands here. If you're looking for map enhancement, there is now a standalone addon called Leatrix Maps (Classic). If you are looking for Leatrix Plus for Battle for Azeroth, it's here.
Automate quests - Automatically select, accept and turn-in quests. Quests which have a gold requirement are excluded automatically.
Automate gossip - Automatically select a single gossip option by holding down the alt key when opening a gossip window. Banker, taxi, trainer and vendor gossip windows are automatically skipped with no alt key requirement.
Accept summon - Automatically accept summon requests.
Accept resurrect - Automatically accept non-combat resurrection requests.
Release in PvP - Release automatically after you die in a battleground.
Sell junk automatically - Sell all grey items in your bags when you talk to a merchant.
Repair automatically - Repair your armor automatically when you talk to a repair merchant.
Block duels - Block duel requests from people who are not friends or guild members.
Block party invites - Block party invite requests from people who are not friends or guild members.
Block friend requests - Block BattleTag and Real ID friend requests.
Party from friends - Automatically accept party invitations from friends or guild members.
Invite from whispers - Automatically invite someone to a group when they whisper a keyword of your choice to you.
Use easy resizing - Resize the chat frame upwards simply by dragging it.
Hide the combat log - Hide the combat log to make more room for chat windows.
Hide chat buttons - Hide the chat buttons to gain screen space.
Unclamp chat frame - Move the chat frame to the very edge of the screen.
Move editbox to top - Move the editbox to the top of the chat frame.
Disable sticky chat - Disable sticky chat so you can switch between group chat and whispers easily.
Use arrow keys in chat - Use the arrow keys to navigate the editbox.
Disable chat fade - Prevent chat text from fading.
Universal group color - Make raid chat text the same color as party chat text.
Use class colors in chat - Show character names in class color in the chat window.
Recent chat window - Open recent chat history in a copy-friendly window.
Increase chat history - Increase chat history from 128 lines to 4096 lines.
Hide error messages - Hide error messages shown in the error frame (with exclusions for a few important errors).
Hide portrait numbers - Hide the damage and healing numbers shown in the portrait and pet frames.
Hide zone text - Hide zone text (such as 'Ironforge').
Resize mail text - Resize text in the mail frame.
Resize quest text - Resize text in the quest detail frame.
Resize book text - Resize text in the book frame.
Enhance minimap - Rescale the minimap, hide zone information and use the mousewheel to zoom.
Enhance tooltip - Move, rescale and color the tooltip and show the unit's guild, rank (if same guild) and target.
Enhance dressup - Add tabard and nude buttons and hide the character controls.
Enhance quest log - Make the quest log frame double wide and show quest levels.
Enhance professions - Make the professions frame double wide.
Enhance trainers - Make the trainer frame double-wide.
Show volume slider - Add a useful volume control to the character sheet.
Show auction controls - Add controls to the auction house frame to speed up creating auctions.
Show cooldowns - Show up to five beneficial cooldown icons (of your choice) above the target frame for easy cooldown tracking.
Show durability status - Show itemised durability status in the character sheet and show an overall percentage after you die.
Show vanity controls - Show helm and cloak toggle checkboxes on the character sheet.
Show bag search box - Show a bag search box in the backpack frame and bank frame.
Show free bag slots - Show the number of free bag slots.
Show raid button - Move the raid frame toggle button so that you can toggle the raid container frame without opening the raid management frame.
Show player chain - Show a rare, elite or rare elite chain around the player frame.
Show Wowhead links - Show Wowhead links in the quest log frame.
Manage frames - Move and rescale the player frame, target frame and mirror timer bar frame.
Manage buffs - Move and rescale buff frame.
Manage widget - Move and rescale the widget frame (commonly used for PvP scores).
Class colored frames - Show the player and target frame backgrounds in class color.
Class icon portraits - Show class icons in the portrait frames.
Hide gryphons - Hide the gryphons shown either side of the main bar.
Hide stance bar - Hide the stance bar.
Graphics and sound
Disable screen glow - Disable the screen glow to reduce nausea.
Disable screen effects - Disable the grey screen of death and the netherworld effect.
Set weather density - Set the density of weather effects.
Max camera zoom - Increase the maximum camera distance to get a bigger view of the action.
Enable viewport - Show customisable borders around the game world with optional screen resize and transparency.
Silence rested emotes - Automatically toggle emote sounds when you enter and exit rested state.
Disable bag automation - Prevent your bags from opening or closing automatically when you interact with a merchant, bank or mailbox.
Show character addons - Sort the addon list according to character by default.
Disable loot warnings - Hide the warnings that appear when you choose a loot roll option or attempt to sell or mail a tradable item.
Faster looting - Loot creatures significantly faster.
Faster movie skip - Cancel cinematics without being prompted for confirmation.
Stand and dismount - Automatically stand or dismount when an action is prevented because you are seated or mounted.
Show vendor price - Show the vendor price in item tooltips.
Combat plates - Show nameplates automatically when combat starts.
Media player - All of the game music tracks and playable cinematics in an easy to use, categorised and searchable media library.