Once you've investigated various healing specs and have your priest adequately geared for raiding, the next step is getting the right add-ons for the task. There may be some classes who can raid with a bare minimum of mods, but healers cannot. If you want to be effective as a raid healer, you will need to utilize add-ons. You've prepared your character, now it's time to prepare your UI for raiding, so you can cast your spells quickly and effectively.
There are a number of mods that are currently popular with raid healers. One popular mod is Grid. In a compact area, it shows all members of the raid and their state of being: their health, any debuffs, HOTs, etc. It is very lightweight, so should not cause lag and can be customized with a number of different modules. You click on the name of the person in the grid that you want to heal, debuff, etc. and hit the appropriate key. While initially a bit challenging to configure, this mod is very popular with healers. There are a number of good modifiers for this addon, which make it even more useful. Recommended additions to Grid are: GridIndicatorSideIcons, GridManaBars, GridSideIndicators, GridStatusRaidDebuff and GridStatusPWShield.
Another popular mod is Clique. This is often used in conjunction with Grid, as it allows click casting with the mouse. You can bind different spells to different mouse click and keystrokes. You can set up this mod so if you left click on a party member it casts Flash Heal; but if you shift-left click, it casts Great Heal. It also is designed such that left click to a friendly target can be configured to cast a the beneficial spell linked to the left click, while a left click on an enemy will cast the harmful spell linked to it.
An alternative to the Grid/Clique combo is Healbot Continued. This combines in one mod, the features of Grid and Clique, but can be memory intense. If your computer can handle it, Healbot is a good choice. It shows all party members in a compact interface and shows their state of being. Do they need buffs? Are they diseased? Are they under a spell? Are they out of range? Is there a Prayer of Mending sitting on that player? Is your renew still ticking on them? It also allows click casting and while it comes with a default setting, it is easily customizable.
A new unit frame healing mod that is gaining in popularity is VuhDo. It can be configured for click-casting and gives extensive information on health, incoming heals, buff and debuffs. The author of this program is very active on the PlusHeal forums and is very responsive to user concerns. This mod seems to be an up-and-coming addon for raid healing and is giving Grid a run for the money in popularity.
While not really a mod, mouseover macros are increasingly popular with priests. With 3.0 the class lost its ability to use down-ranked spells effectively so stopping a cast is increasingly important. Mostly priestly macros begin with /stopcasting and can be used to constantly keep a heal in the pipe, but uncast until needed. By adding the ability to merely mouseover, rather than having to actually click to cast, minuscule amounts of time can be saved, but all those fragments of seconds can add up. Find some good mouseover macros for priests at WoWWiki under Useful Macros for Priests.
Since speed is of the essence in raid healing, you'll want to keybind your macros. You can do this manually or there are some keybinding mods to help you with the process. MyBindings2 is one such program, which is easy to use. Those with G11/G15 keyboards can download a mod that will keybind your macros to your G11/G15's additional 18 keys. * The original G15 had 18 added keys. Currently the 18 additional key model is the G11, but both work with the G15 Buttons (continued) mod. *
Macros can be used with any unit frame. I use Grid. A good general unit frame that can be customized for healing is Pitbull and I know it is popular with many of our raiders, including healing priests. Ag_UnitFrame is another popular choice among raid healers.
Visual Heal is another mod, which can be valuable. "Whenever you cast a healing spell a bar will appear, which shows the health of the target, the incoming heal from others that will land before yours as well as the healing you are about to cast. This makes it extremely easy and fast to determine whether or not the heal is going to overheal." This mod uses LibHealComm-3.0 so it should send incoming heal information from other raid healers using Visual Heal, PitBull, BunchOfBars, Grid, PerfectRaid and sRaidFrames because all of these mods use LibHealComm-3.0 to communicate healing.
Event Alert signals when various talents proc. For Priests, it shows Borrowed Time, Martyrdom, Serendipity and Surge of Light. This mod plays a sound and pops an icon up on the screen, so you don't waste mana-free heals and other important procs. This mod has not been updated for 3.2 but is working if you select Load Outdated Add-ons at the character screen.
A Heads-Up display can be helpful for a priest by arcing important information around your character. It is very nice for priests, as it helps focus your attention on your character. I think all healers have a tendency to get their attention riveted on healing bars and tend to not notice the green goo, or the glowing red circle that they are standing in. With a good heads-up display your information is displayed in an arc around your character, which helps focus attention on you character and hopefully will alert the healer that he or she needs to MOVE! Two well-regarded HUDs are DHUD3 and IceHUD, which have both been updated for 3.2.
If the unit frames you use for healing cannot be configured to show debuffs, you will definitely want to get the Decursive add-on. This can be configured to show who has a magic or disease debuff. Removing a disease or dispelling debuffs are important in many fights and this light-weight mod will identify those in need of help and gives you fast, one click debuff removal.
What mods do I use personally? For raid healing, I use Grid with keybound, mouseover macros. I also use Visual Heal and the DHUD3 heads-up display. In fights like Yogg, where dispelling/curing is of the essence, I will use Decursive, though mostly I use Grid for this purpose. I have a G11 keyboard and I use G15 (continued) to give me a visual representation of the spells I have bound to those keys.
Of course all raiders will want general raid addons like Deadly Boss Mods or Big Wigs, Omen, oRA2, Buff/Debuff bars, etc. Yes, there are some who prefer bare bones healing and a skillful player can heal effectively using the game provided UI and keybindings. However, I think most will agree that the prudent use of add-ons can enhance the game play of even the most skilled player, to the benefit of the entire raid.