Scrolling Combat Text - Just like it says, it adds text over your characters head in combat. Really useful if you are a warrior/paladin, as it tells you when to use overpower, execute, etc. Makes everything easier.
Titan Panel - Useful bar full of info like bag space, gold, exp gained/hour, latency..
GoodInspect - My personal favorite, lets you inspect people from a MUCH farther distance, so you don't have to stand 3 feet away from someone to see their gear.
Those are just addons, the UI mod I use is
Nurfed UI. It's a pretty simple mod compared to some out there, but it allows for a lot of customization and I think the unit frames look nicer then the default ones.
That link is actually a compilation of the nurfed stuff, and a bunch of other mods as well. The ones I listed above are all in there. You can enable or disable whichever ones you want if you install that package.