Priest PvE Healing
This is generally my favorite spec for healing, its isn't going to get you the highest heals but its great for endurance and clearcasting as a priest makes my panties wet.
PvP healing
This spec I find has great survivability. Martydom and Pain supression are just awesome abilities for healing in pvp. The silent resolve points can be tossed around into absolution and mental agility but in arenas its nice to have that chance that people wont immediatly purge a shield.
PvP Shadow
No threat reduction just +meltface I find this spec seems to work best in any given situation. Though hunters may give some problems because of the lack of shadow reach. Its really up to you wther you prefer imp mana burn or the range of shadowreach.
(Solo leveling is this spec with blackout traded for spirit tap.) PvE Shadow
Threat reduction, mana effiency and range. The only thing I think this spec really lacks is the -chance to resist. But its hard to choose which points to take out. Silence and imp fear are the first choice but I like to keep them for pvp. Really up to you on how far you want to devote yourself to PvE.
The lol omfg wtf spec (AKA Power Infuse shadow)
I've never seen anyone besides myself use a spec like this. Why? Well its good at nothing specific but for support in BG pvp its ****ing amazing. Say we're on Van/Drek in AV, I put my healing gear on and can heal fine. Say we've got a priest backing us up in WSG, I can put on my damage gear and provide VE healing. I've only ever gotten compliments on this spec.