In Update 21, players are given a free Classic D&D Red Box on their first character to log on to the game, in each account.
This box contains 6 random cards, a Magical d20, an Eldritch Card Cruncher, and a Classic D&D Anniversary Bag.
Once per day, per account, you can roll the Magical d20 to earn a free card. All cards and items go into the Anniversary Bag.
You can combine two different cards for a variety of rewards. All the cards are unbound, freely trade-able and auction-able.
Link to Wiki page showing what you can get:
Note that some of the rewards are unbound while others are BtA or BtC.