This thread serves as an index to all the noteworthy tutorials dealing with Macros and Interface customization.
Please refrain from discussing any of the linked tutorials in here, use the respective tutorial's thread for that.
Creating A Clean UI - A video that deals with the concept of creating a clean user interface.
Custom Textures - Making custom textures with Photoshop, adding them to eePanels.
Grid shows debuffs! - Simple tutorial on how to show debuffs via Grid along with a list of common raid debuffs.
Power Auras guide - Guide for how to use and configure Power Auras
Power Auras - General Guide and Support - Another general guide on Power Auras support
Backing up your interface - How to create backups or copies of your Interface, including copies to different accounts.
List of interface compilations - A list of most well-known interface compilations.
Backing up your interface - so you don't rage when your WoW crashes and your UI is gone
5.0+ maximum graphics settings - get the most out of your client!
Power Auras for Cataclysm raid bosses and PvP
evn's WeakAuras setup guide
Making a simple UI with kgPanels
Macro syntax guide
Using posted code snippets
How to get a list of all installed addons
Short overview of the "Protected functions" system - What can AddOns automate, what can't they automate, and why they can['t] automate it.