I have several addons that I use for healing, monitoring buffs, and determining threat.
For Threat, I use Omen. I recently switched from KTM on the recommendation of several blogs and forums. I'll be watching it closely to see how it performs, but right away, I like the fact that the screen edges flash red when I gain aggro and I like the Red text message when I pass 90% of the tank's threat.
For Buff monitoring, I use CT_Buff. It's nice to see the remaining time, in minutes, of my buffs, as well as seeing a visual shrinking bar. My only complaint is, that in a raid with lots of buffers (buffers....lol, I hate buffers), the list gets pretty long and monitoring the buffs you're responsible for becomes more challenging.
For Healing, I use two addons that make my life a WHOLE lot easier and make me appear to be an uber healer...lol.
The second addon I use is Decursive. This addon places a little box for each raid/party member on the screen. If the little box turns RED, that means a left click is required on that box. If it turns BLUE, then a right click is required on the box. Now, recently, the spells changed buttons, but the colors remained the same. Currently, Dispel Magic is mapped to RED, and Abolish Disease is mapped to BLUE. Used to be the other way around, but since the left/right click is still mapped to the same color, my muscle memory is still valid.
The first addon, for Healing, that I use is Healbot Continued. I can't say enough about how important this addon is for me to heal effectively. Sure, I can click like a madman, write macros out the yingyang, and fight against my uncanny ability to click the empty space between two buttons on the casting bar. Or, I can use Healbot. I chose the EASY button.
Healbot, how do I love thee? Let me count the ways, in no particular order:
1. Healbot replaces (optional) the standard party frames with nice big green bars representing health. This is one of the nicest things about it; no more squinting at health bars to see who's dying. It also adds a thin bar beneath the health that represents mana/rage/energy and a thin red bar above that indicates who is being beat upon.
2. Healbot is very nice for soloing. As a priest, I was always casting things upon myself: PW:Shield, Renew, etc. To be able to do so without changing my targeted mob is much faster than targeting myself, casting the spell, then targeting the right mob again.
3. I love being able to keep one mob targeted while healing myself or my party. During my Kara run this week, I was in charge of shackling a mob. No problem, I targeted the mob, cast my shackle, and proceeded to heal my assignments all without clicking off of my shackle. I could see my default shackle timer right on the mob and could reshackle at 10-15 seconds on the counter.
4. Healbot encourages muscle memory healing rather than speedy mouse movement and clicking. By mapping healing spells to certain key combinations, and practicing with them, healing becomes much more reactionary and faster. Now, in order to throw that Renew on the 'lock that's killing himself, I just press Ctrl+right click on his bar. Bingo! 'lock has a Renew, and I go back to nose picking in the 5SR. (BTW, I use a Nostromo N52 gamepad and have mapped the Shift, Ctrl, and Alt keys to my pinky row).
5. Healbot places HoT and Prayers of Mending(frisbee) icons at the end of party members' healthbars when those spells are active. The HoTs also slowly fade as the spell runs it course. I can always see where my frisbee is, which means I know to recast it when that pesky 'lock gets it and refuses to pass it on!
6. I can see the whole raid and not just the people in my group. Huge! If I'm on the OT and the MT healer goes down, it's very easy for me to switch to the MT and continue healing with the same style of healing. I don't have to switch to another method of healing due to that person not being in my group.
I'm sure I'll think of other ways that I <3 Healbot, but this post is already a little novelette! If you decide to try it out, make sure you take the time to either learn the default key combos or, do like I did, remap the spells to the different key combos. Also, I've found that remapping the Shift/Ctrl/Alt keys to the column that is reachable by my left pinky makes it much easier, mechanically, to access the different key combos.
Lastly, practice, practice, practice. The hardest part of getting up to speed with Healbot is creating that muscle memory, but once you have it down, healing becomes much easier!