I have a new addiction, it is an online fantasy card-game with strong strategy elements -- pardon the pun -- the game is called Elements and can be played here:

http://www.elementsthegame.com/

There are many different cards in the game (several hundred), but you only play with a limited subset 30-60 that you choose to build your deck, from which you pull randomly during a game. There are many combinations and synergies between cards, so it takes deep strategy to design and build a good deck that can beat various opponents.

There are 3 major modes:
1. against AI with 5 levels of difficulty
2. multiplayer
3. something in-between, where players submit decks to the system (arena) and other players can play against the decks as controlled by the AI. Then the decks are ranked based on win/loss counts. So you can let the AI play the decks you designed against human players.

A single game is only a few minutes long, but you need to play many thousands of games to get good and also to collect the cards as you start out with a minimal starter set and then you need to "earn" the rest by winning games.