If you have Questhelper, you can simply type in /qh incomplete and it knows exactly which quests you've done and which you still need, and will put a symbol on your map where the quest giver is that you've missed.
Gershuun @ Borean Tundra US - Interface & Macros Moderator
There's also EveryQuest - which is just a catalog of, you guessed it, every quest in the game, and it can get your completed quest list from WoW, and then tell you which quests you haven't done - including where they start.