RIFT. In a post to the official forums, James Nichols explains that the game's addon API is currently being tested for implementation, with the hope that players will be able to customize the game's interface while still maintaining the high level of quality found throughout the game. A precise release has not been stated, as the API is still in its earliest stages of development.
Fan reaction has been mixed, as addons have long been a contentious element of many games -- some players feel that they strip down complexity of play, and there are obvious comparisons to be drawn with World of Warcraft. At the same time, the promise to allow free customization and alteration to RIFT has many players excited and looking forward to the new feature. Either way, it's coming to the game, so only time will tell what the implementation will bring.
[Thanks puremallace for the tip!]