Minecraft, but you're unsatisfied with the super simplified, free-to-play Minecraft Classic, PC Gamer's willing to give you your first hit at no charge. The site recently uploaded a demo that will allow you to create a world and shape it as you see fit, locking you out after 90 minutes of playtime.
You can keep creating worlds ad infinitum, provided you don't mind the time restraint, though 90 minutes isn't enough time to get much of anything done in Minecraft's cuboid world. Seriously, we've blown an hour and a half on building a mere treehouse. Which we keep in the backyard of our mansion, which took a team of eight contractors two-and-a-half months to complete.