I've written an addon but it's been difficult to get any more feedback since the initial round a few years ago.
Darkmoon Tracker helps you manage darkmoon cards of the current expansion by scanning the auction house for you, as well as helping to keep track of how many of each card you have in your bags.
As a bonus, I did some fun math to tell you how much it would cost to buy the remaining cards off the auction house given the cards you have for each deck.
I'm fairly new to addons, this was my first serious one. I'd love to get some more feedback on how to make it either more useful or more friendly.