Shadow Era is a pretty new digital Trading Card Game, available on many platforms, e.g. Android (which I use), iPhone, web (which I use), as downloadable/standalone game etc. It is similar to Magic the Gathering in play, maybe a little simplier (from what I have heard, have not played Magic myself).
You buy cards, construct your deck(s), play and win so called Shadow Crystals and Gold (so that you can buy even more cards) and so on. You don't have to buy cards with real-world dollars, but you could. And if you do, those cards are still cheap, comparing to physical products.
Nice graphics, smooth/quick gameplay, plenty of cards, ongoing development with more cards added regularly, possibility to play versus AI or real players, and best of all, you can play the game on the run, in your smartphone! I have been looking for such kind of game for a while to pop up on the Android Market, and now it is here!
Very nice game that I will continue to explore! Highly recommended if you like TCGs!