Duel Network isn't to my liking since people often argue about rulings and effects.
Then I found the recently released YGOPRO Dawn of a New Era which features not only automatic duels but also online play and tag duels(2vs2 similiar to Tag Force on PSP). So people who didn't want to pay for Yu-Gi-Oh! Online 3 or can't find people irl this is a good opportunity!
Also every card is unlocked from the start; make your own deck and start right away.
full automatic rulings
over 5000 cards
tag duels (2vs2)
duel puzzles (win in 1 round in a certain way)
customization (of background music/wallpaper/card sleeves)
Keep in mind this is a game made from fans for fans and is constantly growing.
Download it (around 200mb). Unrar it and then use the "Yu-Gi-Oh! The Dawn of a New Era Launcher.exe" to start the game. No installation required.
You should consider to register yourself if you want to chat and participate in ranked games. Not required tho.
Now click on Play and 2 windows should pop up. One is the Chat window and the other is the Host List(duh). All the buttons should be self-explanatory. Write your nickname (non-registered people too) at the top of the Host List.
Now if you plan on hosting a game you should change the Allowed Cards to TCG/OCG and change the Duel Mode accordingly. Also write a Game Name.
You can also use this topic for Deck Discussion.
FFXIAH / YGOPRO
Katashuro / Skerxan