Blizzard's MOBA. Been playing since I was allowed into Closed Beta through my bnet account a few months ago. It was officially released today (6/2/15). New hero today is a "Warrior" tank with lots of CC named Johanna from Diablo lore.
They are also doing a few cross-promotional giveaways :
Mechano-spider mount for HotS: Have 4 facebook friends (doesn't matter who) and complete this after 'liking' Heroes of the Storm on FB-
Hearthstone card mount for HotS: Win 100 games in Hearthstone
Ironside Direwolf mount for HotS: Have a WoW account with a level 100 character
Heroes of the Storm card back for Hearthstone: Reach player level 12 in Heroes of the Storm
Grave Golem pet for WoW: Reach player level 20 in Heroes of the Storm
The rewards are retroactive, so you get the reward if you've already done the work in wins or levels.