Depending upon which platform you are playing on, there is School of Dragons, School of Dragons: Alchemy Adventure, Dreamworks Dragons: Wild Skies, Dragon Training Legends, Outcast Attack, How to Train Your Dragon, and How to Train Your Dragon Two.
School of Dragons and School of Dragons: Alchemy Adventure are both offered for mobile platforms only. They are initially free but contain in-app purchases. School of Dragons: Alchemy Adventure is more of an adventure game than an action-based game. Dreamworks Dragons: Wild Skies, Dragon Training Legends and Outcast Attack can all be found online for free at Cartoon Network's website. They only require an internet connection and a web browser. How to Train Your Dragon and How to Train Your Dragon Two are both only offered on console gaming systems (Xbox, Playstation, Nintendo Wii and Nintendo DS). The game Super Star Kartz by Dreamworks features the characters from the film How to Train Your Dragon, but it is less about dragons and more about racing.