http://www.pokemontcg.com
The Pokemon TCG Online beta is officially open.
This is a browser based game, which as of right now just has the three starting decks: fire, grass, and water.
I've just played the first match against the computer, but so far this is really fun. I haven't played the TCG in years, but this feels exactly like what I remember the game being like.
It's very similar to Magic the Gathering: Duels of Planewalkers in how the control mechanics work.
I'm not really sure what the pricing model is going to be like. I think it's going to be free to play where you can buy new decks or new cards or something, but I'm not sure.
As of April 6th if you buy the new Black/White cards (in real life), they'll come with a digital code to add them to the online version of the game.
I'll probably buy them just to have.