I just had an idea for a feature that I think would work really well with this website and be very popular and useful. What I'm proposing is a "card tracking" feature that allows you to track which decks of yours contain particular cards. I think this could work being implemented into the current collection manager interface, or it could be a whole new section of the website on its own.
It's very simple how it'd work. You simply type in a card name, and you're told what deck(s) have copies of that card, and how many copies in each deck. You could also make it fancier by having it tell you exactly what version of that card are in each deck (so telling you what set, foil status, etc). Another thing to improve this feature would be the ability to search particular folders if you wanted (instead of all decks). That way, you could keep all decks you actually own in one folder, the others in another. Then, when you search, you can search in the correct folder and get only relevant results.
I think this would be especially useful for players who own less copies of card than the amount they need for all their decks (and therefore need to share between decks). This happens frequently with commander players. It can be hard to remember where to find these cards, but just knowing what decks it could possibly be in will save time in the search.
Please let me know, guys, what you think of this idea!