The. Best. Free. Game. On. Steam. Period. Not free to play. Free.
A deep and complex, at times daunting experience. It is frustratingly hard, yet very rewarding, with expansive and excellent writing, a large open universe, and a lot of variety. You can play through the story and work your way up to being a hero of the galaxy, you can ignore the story altogether and be a merchant or pirate, you can amass a massive fleet, comandeer alien technology, even invade and demand tribute from almost every world in the game universe. It has challenging content from the moment you sign your captain's name on the mortgage on your first ship all the way to the post-story end game, and a game universe that changes based on how you play it. If you launch a large fleet of cargo ships, you will be attacked by pirates. If you repeatedly attack a certain faction they attack you on sight. A vast array of ships, weapons, fighters, and various other systems allows for a lot of customization. As you play through the story, sections of space once peaceful to you will turn hostile based on your affiliation, and while it is not yet complete it is regularly updated and is already a very fun and full experience, and what might become a massive and enthralling universe.
It does not have the best graphics in the 2D space game genre, but what it lacks in visual polish it makes up for in depth of gameplay that is simply astounding for it's price point and 2D format.