Rift brought addons to the game last summer. After some problems in November when addons had to be disabled, they apparently work again. There are 83 addons currently at Curse.
http://www.curse.com/addons/rift
Here are some some popular addons.
http://forum.mmosite.com/thread/2/127/20111213/Rift_Addons_Collection-
4ee84a10f22dc9613-1.html
Here's how to download and add them to your game.
http://forums.riftgame.com/rift-general-discussions/general-discussion/261083-installing-addons.html
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