just an FYI, Spartanui has a fan update on Wow Interface that works just fine with Cata

There's a few out there, I've been using nUI, mostly because I got tired of having to do what Jonathan mentions and I didn't know the copy-pasting trick. It's alright, I like some parts of it and don't like others.

TukUI is nice and simple, and pretty popular.

+1 for TukUI. I use the ElvUI modification of it, it's much simpler to configure.

I'm using ElvUI too, and let me tell you, it's the first UI that I've never felt like replacing.
I've used plenty before from Spartan(well, tried.) and Diablo3Orbs(which I liked a lot in fact, but some of its functionality was not really up to par) to ShadowedUnitFrames and Pitbull.

So go and check it out :)

TukUI for sure. I switched from SpartanUI after using it for about 2 years to nUI but felt it was too cluttered. I found TukUI about 6 months ago and have no intention of switching to another UI (except maybe an edited version of it)

http://tukui.org/blog/

TukUI feels a bit too...proffesional, it's difficult to change anything and it doesn't feel very WoW-ish in it's visual style. =[

I might add, with the newest version of TukUI (v 13), you can configure just about anything in game now with "/tukui". No need to open config.lua or any of the modules unless there's something really advanced that you wanna do

i love TukUI, I've tried moving away from it, but its just too much of a hassle and TukUI works too well

Another +1 for Tukui and the ElvUI variant. Elv is simple to modify and adjust just about anything on the screen. I can't give enough applause for the support base that Tuk has on the site.