https://secure.howtotrainyourdragon.com/play/tournament

"Collect digital Dragons Cards and play them against other Dragons! Become the Ultimate Dragon Challenge Trainer by reaching level 5 and collecting all the cards!"

To play, you need to sign up with Dreamworks.
Card game codes are found throughout the website and are used for: gold, a card, or valuable experience points.

Edit: Apparently you can only play 3 challenges a day. There is also a training center to track your progress.
You start off at level 1 with 300 gold and the Flaming Monstrous Nightmare card. Its stats are: 120 power, 90 stamina, 80 agility, with a bonus of fire coating.

Edit 2: "Today's challenges from Hiccup - Mayhem on the mountain - Dragons have been spotted racing along Lava-Lout. Do you have the fire in your heart to best them at their own game?"
There are two options. Challenge - Earn valuable experience & gold. Practice - Test your skills before challenging an opponent.

Edit π: When you go against a dragon, you select your card. When you play, it loads some then tells you whether you won or lost. I lost 3 time in the practice against: Meat lug, a Changewing, and a Whispering Death. I won against another Monstrous Nightmare. The Nightmare also cannot beat Stormfly, Barf and Belch, and a Scauldron. Of course this is a level 1 Nightmare. I assume that when you rank it up it will be able to defeat them. The game also hints on buying Meatlug, suggesting that you have enough cash to get it.

HICCUP35G
FIVEYEARS5G
BOULDER25E
TZTH15ES
ASTRID35E
MYSTERY20E
LVL5LIH
DRA10EXP
STRIKE15E

HICCUP35G - 35 gold
FIVEYEARS5G - 5 gold
BOULDER25E - 25 experience
TZTH15ES - doesn't work. Proof it is real.
ASTRID35E - 35 experience
MYSTERY20E - 20 experience
LVL5LIH - doesn't work. Proof it is real.
DRA10EXP - doesn't work. Proof it is real.
STRIKE15E - 15 experience