Its called Dueling Network, one of the best sites to play Yu-gi-oh online and also one of the very few that are easy to use and are free to the public, so you wont have to worry about buying some membership or items like some other websites that I have visited require you to do.
THE GOOD STUFF
There are many benefits from using this site, first of all youll notice that you have almost every card in the game available to be used in your decks so youll be able to create basically limitless amounts of decks easily and without needing to unlock them. This site is also very good in order to test your real deck against players all around the world to see how good you are in the game and also in order to build decks that you are planning on buying but before you do testing them on the site to see if its really worth it to buy them.
Some people like me like to make their real life decks into the game and then adding cards that you dont have in real life to it so that you can see how well they perform in your deck and maybe consider buying them someday.
THE BAD STUFF
The major problem with this site is that you have to do everything like you would have to in real life meaning you are responsible for carrying out the effects and requirements of the cards by yourself and you have to lower your own life points according to what the opponent does. Due to the including of a chat feature in the game its easier now to make the other player understand what you are doing, still this system allows for many flaws in the gameplay and leaves too much to a duelists moral code.
The other huge problem with this is the amount of bad players you are bound to run into when playing this game, lots of people try cheating with things like drawing cards when you are not seeing, refusing to lower their lifepoints or summoning monsters without fullfilling the conditions. And most annoying of all is the fact that when you pull out a winning card they quit most of the times ending the duel in nothing.
For all those duelists out there that really know the game this game is definitely something they must get into to test their decks and further improve their gameplay however for those noobs that know nothing about it and just want to annoy and cheat others please be so kind as to leave the game.
Cyber Dragons FTW