FreeOrion

FreeOrion is a turn-based 4X strategy game for players who believe that controlling one planet is simply not ambitious enough. Inspired by the Master of Orion tradition, it puts you in charge of a space-faring civilization with the usual responsibilities: explore the galaxy, colonize worlds, research technologies, negotiate with alien civilizations and occasionally solve diplomatic disagreements with enormous weapons. The game is open source and places considerable emphasis on strategic freedom, allowing different approaches to victory and empire development. Its interface may take a little patience to master, but beneath it lies a substantial strategy system. If you enjoy games where a single innocent decision can eventually result in a galaxy-wide war involving seventeen species and absolutely no sensible reason for it, FreeOrion has plenty to offer.