Explore other star systems. Earn money by trading, carrying passengers, or completing missions. Use your earnings to buy a better ship or to upgrade the weapons and engines on your current one. Blow up pirates. Take sides in a civil war. Or leave human space behind and hope to find some friendly aliens whose culture is more civilized than your own...
Endless Sky is a sandbox-style space exploration game similar to Elite, Escape Velocity, or Star Control. You start out as the captain of a tiny space ship and can choose what to do from there. The game includes a major plot line and many minor missions, but you can choose whether you want to play through the plot or strike out on your own as a merchant or bounty hunter or explorer.
Endless Sky is a free, open source game. The source code is available under the GPL 3 license, and all the artwork is either public domain or released under a variety of Creative Commons licenses. (To determine the copyright status of any of the artwork, consult the copyright file.)