So...back in vanilla WoW (in Naxx) some folks wrote a mod called HealSync that, in a raid environment, would show the casting bars of all incoming heals on your target (provided the casters had the mod as well). There were a number of mods that attempted to serve this function, and I can't find one that works. Did these get permanently broken with the new UI changes, or are they still out there and functioning run under my nose? As a kid my Mom would always "find" things right in front of my face, so either's possible :P
It doesn't require other people in the raid to also use it which is quite nice.
Quote for truth. This mod will show incoming heals on yourself, your target, and your focus, each in seperate frames. It's incredibly useful even if you're not a healer for the incoming-self feature; it'll show you if a greater heal is about to drop on you, saving you from blowing a bandage or healthstone, etc.
I believe RDX has recently added this to the latest releases
Yes, the heal-prediction functionality just added to RDX is oustanding. I don't think I have a good screenshot handy. Anyway, I'm not here to plug RDX (but if you do use it, you get to use my awesome bossmods).
Any addon that does this will require all healers to be running it. Even though the 2.0 API includes methods which let you see the cast bar (including which spell is being cast) of an arbitrary unit (friendly or hostile), it not provide the target of the spell. Since you can now cast a spell on something without targeting it even momentarily, there is absolutely no way for your client to consistently know whom others are healing.
X-perl unitframes also has the option to monitor cast times, although it can monitor all casters not just healers, nice for letting me know which mages to talk to about stopcast macros to speed up there casting :P
I personally use and prefer VisualHeal (http://ui.worldofwar.net/ui.php?id=3749) - cleanly implemented, works quite well, and provides both heals-on-target (for healers) and incoming-heals (for the rest of the raid) - for the former, it'll show any heals scheduled to land before yours, then your own heal, in a 3-bar stacked format (current HP, total heal before yours, your heal). For the latter, it shows current hp, next incoming heal, and then the sum of all other incoming heals.
(Of course, for predictive-heal display, the other members of your group must also be running VisualHeal.)