Mages can take advantage of a wealth of User Interfaces. There are several websites; I have found that most of the UI's I use and need are located pretty much at one of a handful of websites. Mostly they are found at Curse-gaming.com
World of war.net has a great site that has a lot of UI's on it as well. Best thing to do really is to search out for what your looking for.
UI NAME: Cryolysis
Download: Cryolysis Most Current Version
Without a doubt the Most useful addon for mages is Cryolysis it has gone through numerous patches and rebirths... Essentially what it does is allows you to hide many of the spells we have, but still have them easily available. The Teleportation/portal spells, conjure water/bread, Create Gems(all of them), all of your personal buffs, the list goes on... GET THIS!
UI NAME: MageEye
Download: MageEye Most Current Version
This is pretty usefull as well, it's a bit of a pain in the butt to let everyone know your decurising and sheeping. This UI does that.
Download: ElementalFlux most Current version
ElementalFlux shows the remaining lifetime of a Water Elemental. It can be configured to also show it's cast bar. The frame can be moved with shift click, and resized with alt click.