Some of my personal Faves:
Recount - DPS/TPS/HPS/ all around statistical breakdown of each fight
Perl UI - great UI if you like small, but usable interfaces
Bartender 4 - great bar add-on
Pally Power - should be a standard for all Pallies
Omen - great threat meter
Deadly Boss Mods - shouldnt have to explain this
Atlas Loot - know what you are looking for before you go into the instance
Rating Buster - Breaks down each piece of gear to see your gear maximization