X Perl Unit Frames is a mod designed to enhance the standard unit frames elements of the WoW UI, including your characters portrait, your target, your target's target, as well as party and raid interfaces.

These enhancements come both in the form of visual improvements, such as displaying a 3D animated "head and shoulders" portrait of the character, but also additional functionality such as displaying your/your target's cast bar as part of the frame, debuff/aggro highlighting (eg: players with aggro in the party/raid interface are highlighted with a red border) and more.

Also, it allows for a much more customized UI, allowing you to position each unit frame how/where you want it (the party frame is a single element, as are the 9 groups within the raid ui), as well as change colours, sizes, appearance and what information should be displayed.

I started using this mod a month or so ago and now definately couldn't go back - in my opinion not only does it look better and cleaner than the standard UI, but also I like being able to customize it to display what I want, how I want.

You can find out more information and screenshots as well as download this UI mod from here.