With Hearthstone: Heroes of Warcraft, Blizzard is tapping into a burgeoning market: free-to-play collectible card games. For the company behind World of Warcraft, it’s the merging of two of the most lucrative game types around. Digital versions of CCGs consistently rank among the top-grossing apps on iTunes and Google Play, while browser-based titles like Card Hunter have been able to take the best of physical games and translate them to the web. Hearthstone has been in closed beta for a few weeks now, available on both Windows and Mac (an iPad version is also in the works). In traditional Blizzard fashion, it's a game that manages to blend accessibility and depth into a frightfully addictive package — and a lot of that has to do with the move away from the physical constraints of a typical card game.

"In some ways it was actually freeing," says lead designer Eric Dodds, "because we took a step back and, instead of taking a physical game and creating a digital game based on it, we weren't tethered by any of those things that work wonderfully in the physical space but don't work so great in a digital space."