Addon Control Mods

ACP sylvanaar

Allows players to turn on/off mods and reload their ui in game instead of needing to log out. Also lets players make sets of mods.
Curse, WoWInterface
Review: WoW Insider

Addon Loader Ammo, Cladhaire, mikk

AddonLoader is an addon that loads other addons automatically, as and when they are needed, which speeds up the logon process. Note that addons need to be aware of AddonLoader for this to work; if you see "X-LoadOn-..." tags in their .toc files, they are!
Curse

Ampere Tekkub

A small addon designed to mirror the functionality of the login screen "Addons" button in-game. It's designed to fit inside the config GUI introduced in patch 2.4. Amp lets you quickly enable and disable addons, reload the UI, and load LoadOnDemand addons.
WoWInterface, Curse, github

 
Main Mod Sites
UI Forums
Addon Reviews/Info