Some big changes are coming to the way the UI is coded in patch 2.4, so they set up a proper thread in the forums to document them. Here's what's there so far:

Significant Changes
* The combat logging mechanism is completely overhauled with explicit data instead of text strings delivered to the UI. See slouken's post below for a sneak-peak at some of the current details. Highlights include detailed flags indicating relationship of involved units to player, unique identifiers for units with the same name within the combat log -- great for log analysis.

Visual Settings
* There's a new cVar unitHighlights; 0 = no model highlighting with Alt-Z, 1 = model highlighting on with Alt-Z. (e.g. /console unitHighlights 1)

A very good post on the forums... there are some official UI changes coming in with WotLK including:

- Built-in Boss warnings
- Built-in Threatmeter(s)
- Built-in Itemrack/closet-gnome
- Built-in Mass-mail

And others!

Alot of people want alot of different changes to the World of Wacraft UI, how it handles macros, etc, etc.

This thread attempts to keep you up to date on exactly what has been implemented, what hasn't, and why.

You gotta check out this UI - its sick!

Once again it's time for the ever-increasingly popular Reader UI of the Week. This week brings us something special, in that these teeny-tiny images link to GIANT MONSTROUS IMAGES. This is the widest screen setting I've seen so far. Be warned, clicking opens massive real-sized images that are about 2 megs each.

The WoW Insider guys found this one and they had a lot of praise to give it... in fact I'm trying it out right after this!

This is a big one, and one I've been waiting for for quite some time (about six months, I'd say). I don't usually bother even looking into compilations of mods, since I've found I can always put together a more satisfying solution myself, but MazzleUI may well prove to be the exception. Here's how Mazzlefizz, the author, describes it:

• MazzleUI is a comprehensive, customized user interface for the World of Warcraft that attempts to be clean, symmetrical, provides lots of information, use minimal space and be highly efficient.

Wow Insider have a write up on a purely cosmetic UI addon that makes all your buttons pretty little circles.

CyCircled will apply a variety of textures and styles to your buttons: Sprockets, Hex, Squares, Circles, and your choice of glossy or non-glossy. You can also set the surround colors for the button in different states.

Yes, CyCircles is a purely cosmetic AddOn. Sometimes your UI just has to look sexy for when the friends come by, you know?

I am sure there are other, more practical and helpful UI addons out there, but this one does look very good!