Supercell just soft-launched Clash Royale and you must play it right now. I speak from experience: on Monday, I wasn't hooked to any current free-to-play games. On Friday afternoon, I'm checking the game every few hours to collect my free rewards, unlock my chests that are ready, along with communicating and giving cards to my clan (all glory to Para_Digm and his Beard Nation). This game has become a thing in my life. It's that good, and you need to be playing it.
Clash Royale is real-time strategy meets MOBA meets CCG. You go into battle with a deck of 8 cards, 4 of which are in your hand at any time. Each card has an elixir cost, and to play it, you must have that much elixir, which recharges over time. You can place your units anywhere on your side of the screen, but they will go toward the paths toward the enemy's crown towers, which you must destroy to win.