QuestQuery allows you to search your quest log. Matches are listed in your chat window. Clicking them opens your quest log to the appropriate quest.

All aspects of the quest are searchable, including quest name, zone, description, objectives, and various tags (dungeon, heroic, completed, failed, daily, etc.)

Example usage:

/qq failed            What quests have I failed?  
/qq ashenvale         Do I have any quests in Ashenvale? 
/qq spectral comb     What quest is this "Spectral Comb" in my bags for? 
/qq young raptor      Do I have a quest for these Young Raptors?

Searches are case insensitive. Search strings can be any length, and may contain regular expressions. You could use regex to combine searches, like "shadow.*comp" to find all completed quests in Shadowmoon.

I've been using this addon for years and it's very handy, so I figured I should probably share it with others.

Cheers. :)