Dark Horse: One who was previously unknown and is now prominent.
Dead Ringer: 100% identical. A duplicate.
Devil's Advocate: Someone who takes a position for the sake of argument without believing in that particular side of the arguement. It can also mean one who presents a counter argument for a position they do believe in, to another debater.
Dog Days of Summer: The hottest days of the summer season.
Don't count your chickens before they hatch: Don't rely on it until your sure of it.
Don't Look A Gift Horse In The Mouth: When someone gives you a gift, don't be ungrateful.
Don't Put All Your Eggs In One Basket: Do not put all your resources in one possibility.
Doozy: Something outstanding.
Down To The Wire: Something that ends at the last minute or last few seconds.
Drastic Times Call For Drastic Measures: When you are extremely desperate you need to take extremely desperate actions.
Drink like a fish: To drink very heavily.
Drive someone up the wall: To irritate and/or annoy very much.
Dropping Like Flies: A large number of people either falling ill or dying.
Dry Run: Rehearsal.