Pioneer

Vega Strike   (Compare Project)

Mostly written in C++

License: gpl

Galactic Fall   (Compare Project)

License: gpl

Gravit   (Compare Project)

Mostly written in C

License: lgpl

Speed Dreams   (Compare Project)

License: gpl

DynaEngine   (Compare Project)

Mostly written in C#

License: Unknown

BZFlag - Multiplayer 3D Tank Game   (Compare Project)

Mostly written in C++

License: LGPLv2_1, mozilla_public_2_0

Openlancer   (Compare Project)

Mostly written in C++

License: gpl

Gaia Sky   (Compare Project)

License: gpl3

Stellarium   (Compare Project)

Mostly written in AMPL

License: BSD-3-Clause, cc-by-3, gpl, lgpl21_or_later, Public_Domain

Oolite   (Compare Project)

Mostly written in Objective-C

License: CC_AttributionNonCommercialShareAlike_3_0, gpl