Plugins and mods
Dalamud plugins add features to Final Fantasy XIV while it runs. You install them from the Plugin Installer, which the /xlplugins chat command opens once the game is started through XIVLauncher.
Each plugin is built for one Dalamud API level. When Dalamud moves to a new level, plugins built for the previous one show (outdated) in the installer and stay off until their authors update them.
Mods from Nexus Mods replace the game’s models, textures and sounds on your own client and are loaded with Penumbra or TexTools.
Adds a simple front-end for browsing and changing the in-game BGM.

Play a chime when someone comes into a house.

Chat and game dialogue TTS plugin.
Bard performance assistant

A Dalamud plugin that allows volume control from anywhere, including gpose and cutscenes.
Get aurally rewarded when you get commended on duties.
See Discord voice activity indicators directly on your party list.
Filter any game sound.
Play sounds and highlight text based on defined filters.

Shows the date when you acquired something.
Play OOF! on fall and/or death. The Only Plugin You Need!

Configure sounds for critical and direct hits, plus some countdown music.
You like annoying low HP sounds? This is great for you!
Automatically control mount BGM for individual mounts!

Witty commentary to the brilliant things you do.
Like you'd ever be sick of the in game music right?
Display what is playing in your server bar, and play/pause/skip.
Like /bgm but automatic

Changes your VO language on a per-duty basis
Adds Moonbase Alpha TTS when in Cosmic Exploration. aeiou!
Even the Ascians needed some tunes.

Stop yer ears bleedin' in the Wolves' Den
Listen to radio in Eorzea!

Stream internet radio stations while playing FFXIV
Quiets the Gameplay Experience.

If it's not 'big fat tacos', what ARE they singing?

For those of us too lazy to read their screens.
An actually updated Mumble positional audio plugin
Tag items as important, highlighting them visually. Now with sound!
Generate a crafting list for the faded orchestrion rolls in your inventory
Tweak the built-in 4-band EQ with custom presets.

A plugin that allows you to map hotbar actions to a novation launchpad
URL music player for your mounts! Stream internet radio when on a mount or on foot.
Control what's playing on your computer within XIV!

Guess the tune from FFXIV's orchestrion library