Free to play Collectable Card game by Blizzard

While there is no way for the average player to make any money with this game, there is a Hearthstone sponsored world cup, released in early 2014, the first year's world cup and winner of $100,000 was a 18year old guy. There is also a number of Hearthstone related sites that sponsor tournaments. So If you enjoy the game, there is a chance to earn money if you're good enough.
Free to play on PC, MAC and Android Tablet, you actually get a free pack if you play from a tablet once. Join Hearthstone and send a friend request to "Goozik#1887" I get nothing for referrals on this game, so please just challenge me to a game for some fun.
30 card decks.30hp life pools. Each players mana pool grows by 1 each turn ensuring you both are limited in the power you can play makes for a fun and strategic turn based collectable card game. Each player can switch out any/all of their starting hand for new random cards from their deck helps ensure you start the game with playable cards.
8 Classes of Hero's with it's own unique power you can activate once per turn, each class has it's own set of class cards plus there is a large selection of neutral monsters for any class to use.
3 game modes, PvP, Solo Adventures(vs AI), and Arena (draft a deck).
Class Cards, commons, rare, epic and legendary create a nice mixture of power and availability for everyone to build their decks. Normally you are allowed 2 copys of each card in your deck except for legendary only one of each.
Daily quests for gold ensure you earn enough to play w/out grinding for cards.
Disenchant any card you don't like to create card dust, which you can use to create cards you do want. Different rarity of cards worth different dust.
Online only game means Buffs and Nerfs do happen, but when a card is adjusted you are given some time to disenchant the card for full value.
Release in January of 2014 the game is still new with only two expansion released.

   Solo Adventures:Player vs AI. Only way to unlock cards from first expansion

Practice vs Normal or expert controlled AI for each class. Excellent to learn the game. Defeat every expert controlled class and you are awarded with 100gold. The game has many "hidden" quest you can complete like that, more in depth section on those located farther below.

Curse of Naxxramas (the 1st expansion) introduced new Normal,Class and Heroic challenges for player vs AI mode. Unlock the 5 different sections of the expansion with gold or pay money. Beating each enemy in the Normal challenges awards you new cards from the expansion, in the Normal section you must do this with one of your own decks. In the Class section you are given a deck to compete with and again are awarded new cards for beating each enemy. Note you are only awarded cards for being each enemy once, so you must beat them all to get the full expansions worth of cards. Heroic is just a test of fun and skill, you receive no awards for beating them.

           Arena:Draft a deck and try to win 12 times before you lose 3 times.

Arena mode costs 150 gold or $1.99 real money to play, But your very first Arena run is free. Choose from 1 of 3 classes at the start and then you draft a deck of 30 cards by choosing 1 of 3 cards from a group to add to your deck. You then play vs other people who have drafted decks the same way. The goal is to get 12 wins before losing 3 times. The more wins you get the better prizes. 

Always awarded 1 pack to open plus some additional gold,dust or cards as prizes it's a great way to have fun and build your collection while competing on a almost level playing field.

Pick #1 and #30 are guaranteed to be Rare,epic or legendary. You 3 cards presented will be the same rarity(I.E you choose from 3 rares, or 3 epic etc). There is no limits to deck building in arena runs, you can have more then 2 of same card and more then 1 of same legendary in your deck.

Win 500 games with one class in Arena and you're awarded golden version of that Hero.

Quests can be completed in this game mode.

                   Play mode: Player vs Player. Casual and ranked modes

Casual play is as just for fun, you gain no rank. But you can still complete quests to earn gold in this mode.
Ranked is where the competition gets fun and serious.Start at level 25 Every game you win, you gain a star gain enough stars and you go up in Rank. Win multiple games in a row and go on a winning streak for bonus stars. Once you are rank 20 you will qualify for that months special free card backing(graphic that changes how the back of your cards look). If you ever reach legendary rank(which is after you hit rank 1) you get a special Legendary card backing.
Quests can be completed in this game mode.

                    Daily and Hidden Quests: Variety of tasks for free gold or cards

Every day you are given a Daily quest, you can store upto 3 Daily quests. Gold is also the Daily quest reward and values from 40gold up to 100gold. Quests usually are win 3 or 5 games limited to 2 classes. Although there are some like Cast 40 spells or 20minions that cost 5 or more mana. Quests update when you first log in and after the game with info, but you can also look them up under the Quest log from the main screen anytime.
There is a large number of hidden quests in Hearthstone that usually award gold. I'm going to list a few of the really easy to complete quests below, for a complete list please visit the Hearthstone wiki page on quests.
Get any class to level 10 - 1 Classic Card pack
Complete 3 games in play mode - 100gold
Disenchant your first card - 95dust
Awarded for playing a game on iPad - 100gold
Defeat every Expert AI Hero - 100 gold
Acquire every Pirate - Card Captain's Parrot x2 (Epic Neutral Beast)
Acquire every Murloc - Card Old Murk Eye (Legendary neutral Murloc)
Win 100 games -300 gold