User interface add-ons are plug-ins that change the user interface that a player utilizes in a game. This's where the folks "Curse Client" for Window -- designed specifically to deliver user interface add-ons that 3rd party authors publish through Curse.com comes in.
"Through Curse.com, user interface add-on authors can publish their add-ons which then appear in Curse Client for gamers to download and install. Currently, Curse.com supports add-ons for 3 games: World of Warcraft, Warhammer Online, and Runes of Magic.
One example of a user interface add-on is a GPS-like tool for World of Warcraft called "Cartographer." This add-on places an arrow in the user interface which tells player how far away they're from their target and directs them to their destination. Since the game world in WoW is so large and complex, user interface add-ons such as "Cartographer" are essential to navigating your way around.
Download: Curse Client