Auctioneer (Helps understand and use market trends on the AH)
GatherMate or Gatherer (shows potential spawn locations for resource nodes that you have discovered)
Weak Auras
Power Auras (shows your buffs)
VendorPrice (shows price of quest items so you can pick the one that sells the best)
SellJunk (sell all junk with one click)
a combat mod like
SwingTimer (Warrior), or ShotTimer (Hunter), NugRunner (dot timer for warlock), etc... these are indispensable.