Mage War

From BoardGameGeek:

Mage War is a FREE trading card game where you construct your own cards and make decks to battle other players. It is a game of strategy with a focus on balanced mechanics that put opponents on equal footing. (No need to worry about an opponent having an advantage over you because they spent more money to build their deck.) The rules lay out all the guidelines for designing and creating your own cards and decks as well as how to play. The limits of your power are the limits of your own imagination! Craft your own cards and build your own strategy around your own play style and pit your ideas against your friends. -description from the designer