At last week's Blizzcon, Blizzard announced that all Hearthstone players would net the new Legendary card Marin the Fox for free from November 6th. Consider this your reminder (because I completely forgot, so perhaps you did too).
Marin the Fox is technically part of Hearthstone's upcoming Kobolds & Catacombs expansion, due this December, despite his early release. He's a lovable swashbuckling rogue, and an eight-mana-cost minion that will place a 0/8 treasure chest (known as the Master Chest) on your opponent's side of the board when played.
If you, as the summoner, then destroy that chest, you'll receive one of four treasure cards: Golden Kobold, Tolin's Goblin, Wondrous Wand, and Zarog's Crown.
These respectively replace your hand with Legendary minions, fill your hand with copies of a drawn card, reduce the cost of three drawn cards to zero, or summon two copies of a discovered Legendary minion. And the fun ensues!
All you need do to grab your copy of Marin the Fox is log into Hearthstone right now. Alternatively, Blizzard says you'll get a copy of Marin the first time you acquire a Kobolds & Catacombs card pack after the expansion's release in December, if you don't manage to pick him up for free before then.