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 attributes of the quest are searchable, including quest name, zone, description, objectives, and tags (dungeon, heroic, completed, failed, daily, etc.)
Example usage: /qq completed= Show me all my completed quests. /qq ashenvale = Do I have any quests in Ashenvale? /qq spectral comb= What is this "Spectral Comb" in my bags for? /qq young raptor= Do I have a quest involving 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.
1.1 Updated for WotLK, cleaned up a little while I was there (removed QuestQuery.xml)