This app was mentioned in 5 comments, with an average of 1.20 upvotes
From website: "The creators of Ascension and Magic: The Gathering have joined forces to create a revolutionary digital trading card game. SolForge combines all of the strategy and depth of a traditional trading card game experience with the convenience of online and mobile gaming. Players can experience Solforge absolutely free and can grow their collection just by playing the game. Build your own decks to play against your friends, compete in online tournaments, and earn rewards!"
I bought a Humble Bundle a while back that had starter decks for a number of online card games. That's how I got into Scrolls in the first place.
I also got a few Solforge decks with that Humble Bundle. Deck building is currently broken on their Android port, but the Windows client syncs the decks with their central servers so I can edit a deck by booting up my XP VM. Bit of a pain, but I'm still new to these games so deck building won't be a priority right now anyway. According to their support forums they're having their clients, which were only beta, rewritten from scratch for a Fall release.
I also have decks for Star Realms thanks to that bundle. I even have physical decks for that one. Star Realms is another cross-platform system that syncs decks across platforms and it offers OFFLINE play as well!
Both these systems lack the castle / battle field aspect of Scrolls, but since Scrolls was really the first collectible card game I got into I still don't know who attached I am to the "field of battle" mechanic.