Infiniverse!

Ascii Sector is a free space combat/exploration/trading game for Windows, Mac OS X and Linux. You start with a simple spaceship and can then accept missions or trade goods to earn enough money to upgrade your ship or buy a new one.

Naev is a 2D space trading and combat game, in a similar vein to Escape Velocity. Naev is played from a top-down perspective, featuring fast-paced combat, many ships, a large variety of equipment and a large galaxy to explore. The game is highly open-ended, letting you proceed at your own pace.