Blizzard is launching a free-to-play collectible card game set in the Warcraft universe
Set for release later this year, I expect it'll be like other FtP CCGs in that you'll get an initial deck and a randomized booster and have to pay or win more cards. Could be fun? Could be a laggy piece of luhsa like the Magic: The Gathering IOS game.
What we know so far about HearthStone: Heroes of Warcraft

HearthStone is a new digital collectible card game (CCG) that takes place in the Warcraft universe.
HearthStone is free to play.
It's set for release this year. The beta will happen this summer.
You can sign up for the beta now over at HearthStone's site.
It will initially be playable on the Mac and PC. iPad version to follow soon (and not Blizzard soon, but reality soon).
PC and iPad version are playable now at PAX East.
Blizzard is looking at the possibility of future platforms (in response to the Android comments), but development is focused on PC, Mac, and iPad right now.