Grid - One of the 2 most commonly used raid frame mods. It requires some setup to get it how you want it but it is well worth the time and your options are really endless.
- Priest AoE - This addon has had the biggest impact on Grid in my opinion. It shows you, via an icon, color or text that you place, who is the best target in your raid and each party to cast PoH and CoH on to gain a threshold of healing that you can set. This is absolutely amazing in settings where the raid is somewhat spread out and you, obviously, want to hit as many people in the party as possible, this will help make that happen.
- Mana Bars - It's always a good idea to track the mana of your fellow healers so you can help them out, use Hymn of Hope at a prime time, etc. I suggest making this bar rather skinny so it is not too obtrusive in your quest to keep health bars from reaching 0.
- Side Icons & Corner Icons - Track as many things as you can. PoM, Renew, PW:S, WS, PS, GS, Rejuv, Regrowth, Lifebloom, Riptide, Earth Shield - by tracking other healers' buffs and hots you will become more of a team player within a raid by not, inadvertently, sniping their hots. It is also useful to see, ok this tank has no hots I better prepare to heal him, etc.
- Raid Debuff - This addon, usually, adds raid debuffs automatically to center icon so you are aware who has important debuffs.
- Status Hots - Shows colors or timers via text for hots.
- Heal Trace - Shows via icon, text or color who is being healed by your AoE spells. I have found this addon to be great with CoH and Divine Hymn tracking - I'd also suggest showing the Divine Hymn +10% healing buff as an icon, color or text to one of your available areas to best make use of DH. For CoH it's great to see how many people and who you are actually healing with it instead of just assuming it's doing a good job.
Clique - A great tie in with Grid and other raid frames. This addon allows you to easily create mouseover and click cast combination heals. I highly recommend using at least mouseovers for all of your spells as it can greatly increase your healing throughput and ability to save lives. Yes, in the long run that 0.5 sec you are saving by using mouseovers/clicks adds up over clicking a person then hitting a keybind. Make sure your most commonly used spells such as PoH, CoH and PoM are very easy to hit as well as your important cooldowns such as Guardian Spirit.