The pretty black gradient panels you see on the top right, left and bottom of the screen are made with eePanels2. It seems this addon is no longer being updated, but it works fine currently.
It's a very nifty little addon that you can use to make frames to put your bars onto, add custom art to your UI, funky gradients, etc. I use it in the simplest of ways - I have three gradient panels, very plain, no borders. The ones at the top and left are used as spots for buffs or other buttons, without being obtrusive - if there are no buffs up, they simply look like a nice border. I like the asymmetrical look of them, but you could ditch the gradient look and have solid panels if you wanted.
The gradient at the bottom of the screen is a couple of inches tall, and although I can see through it and it doesn't obstruct my view of bad things at my feet, it makes text and other UI elements easier to see, rather than just have them floating on your screen.
Down in the bottom right are my player and target (ToT, focus etc) frames. For this I use Pitbull.
Pitbull is very, very customisable, and I can't really go into all of the different things you can do with it, because it would take forever. Pitbull is responsible for those funky unit frames you might have seen with the character portrait that runs through the middle of the bar. Very stylish - there are lots of different things you can do. If you spot a sleek looking unitframe on someone's UI, there's a good chance it's Pitbull.
As you can see on mine, I have changed my own frame to green (rather than druid orange). I've left a blank space between my health and mana bars, and inside that I put my cast bar.
On the target frame I have done the same thing, but also asked it to put the target's buffs inside that blank space as well, so I can see my buffs and HoTs on them (this is more for aesthetics than for any practical use, though!).
I find it really cute to be able to put things right inside your unit frames. For example, my old hunter UI had a pet bar that was inside my pet's health bar - things like feed pet, passive/aggressive stance, growl, and mend pet were right on his unit frame. Here's an old version I dug up:
It's a cute way of doing things; but I stopped when I decided I wanted to try and have a UI with absolutely zero buttons showing :P
Pitbull is also used for party frames and raid frames, but I don't use these features; I just use the player, target, ToT, ToTT and focus/focus target frames.
Well, as you can see I don't really have action bars as such; no buttons are visible when I play. But I still use action bars for things like food, trade skills, mounts etc - they are just hidden, and show up when I use modifier keys like ctrl, shift or alt, or simply when I mouse over the spot that I know they live in.
For this I use Macaroon, which is the 3.0 replacement addon for Trinity.
It handles almost the same as Trinity did, although you need to get used to the fact that it uses mouse-scrolling to increase or decrease values (such as button count). That took me a while.. but now I love it.
Like most bar mods, you can make bars invisible as described above, which is handy if you're building an minimalist UI. Obviously if you don't have bars, you'll need to have your abilities bound - I use BindPad for this (see below).
Bindpad is wonderful.
This is something you can't see on my interface, because it works behind the scenes; but if you use any action bar mod then I highly recommend grabbing it.
Bindpad gives you a window into which you can drag spells, items, macros - just about anything really, and then bind them how you like. Here's what mine looks like at the moment:
So you can see I have a bunch of spells in there, all bound. This pic shows that I use G for poisons and shift-G for curses.
Buffs and Debuffs
For buff and debuff displays, I use Buffalo. Buffalo lets you move your buffs and debuffs around, resize them, decide which way they grow, etc.
I chiefly use it to make my debuffs huge liek xbox, under my map, so I can easily spot them. I'm not really concerned with my buffs as much.
I use Simpleminimap to let me move my map around, change the size, border, shape, etc. It's a very simple little map addon. I basically just use it because I wanted a simple way to move my map around, make it square, and change to a simple thin border.
YOU MUST GET QUARTZ.
Quartz is an absolute must for DPS classes, but even for healers it is fantastic. Most importantly, it has a little red section on the cast bar that shows you when it is safe to start casting again to account for lag. Again this is mostly something that DPSers use to maximise DPS and avoid "clipping" shots, but if you're chain casting Regrowth or HT for example, it makes a huge difference if you can tell when the soonest time is that you can begin to cast your next heal.
So as soon as the cast bar gets into the red section, it is safe to cast again and you won't get an error message saying that another action is in progress.
Quartz also has a GCD bar that will show you your GCD; I use this because I don't have any buttons on my screen to show me the GCD :P
Chat: Prat - allows you to customise your chat windows with different fonts, layouts, colours, etc.
Cowtip: Allows you to move your tooltip to any position, and customise the look and what information you would like it to show.
MiniMapButtonBag: Puts all of your minimap buttons inside one, to save clutter around your map.
MSBT: an alternative to Blizzard's scrolling text. Lots of configuration options, different texts, etc.
Outfitter: one-button wardrobe changes! Allows you to set up "outfits" such as healing gear, dps, tanking, fishing, resist sets, haste gear, or even just town sets, without having to dig through your bags to find all of the pieces. Also has a very handy "birthday suit" outfit which will strip off all of your durability gear in the case of needing to suicide. In combat it will remove your weapons to save you a bit of money if you're about to die.
I think that's it!
If there's something you see on my UI that you're not sure about, or you'd like to replicate but you're not sure how, let me know and I'll try to explain what to do, or perhaps do another entry on that particular addon.