Trion is offering a 72-hour trial for Defiance. It's open to all PC users to try out the game and decide whether they would like to purchase the game in full.
To sign up, you'll have to head on over to the Trion Worlds website and register for an account to play the game. The 72-hour countdown begins as soon as you register, so it'll be a good idea to play the game as soon as you possibly can.
Keep in mind that the trial itself is limited—beyond the 72 hour countdown, you're limited to an EGO rating cap of 200, but know that it's possible to carry over rewards, progress, and store purchases should you decide to buy the full version of the game at any point.