Hey!
Recently I've been playing a lot of Hearthstone, an online TCG based off of the 'World of Warcraft' characters and franchise. Not only is the open beat for this game free, but there are also advertisements for them here on the forums. This uses Battle.net, like many other Blizzard games. The game is heavily based on skill, strategy, and luck.
First of all, there are 9 Different Classes: Mage, Warrior, Rogue, Hunter, Shaman, Paladin, Priest, Druid, and Warlock. These each have different abilities, as well as special cards only they can use. Each card costs a certain amount of Mana. Your Mana is restored after every turn, and you gain one Mana crystal per turn, which increases your maximum Mana. The more Mana a card costs, the more powerful it may be.
There are also many other much more complicated things such as Status Effects, and Battlecry, which you can learn from the in game tutorial.
You can download the game, as well as Battle.net from the website here: http://us.battle.net/en/int?r=hearthstone
For those that have it, you may also share current strategies, builds, and Battletags here. For those that want to add me, my Battletag is DeadFantasy