Vega Strike is an interesting space simulation game in 3D, fully developed under the GPL licence (and therefore free), which combines elements of an action RPG with galactic battles against all kinds of aliens and other enemies of the universe. Your task is to travel through space accepting missions using your resources wisely and interacting with other space crusaders.
It is very difficult to get to grips with at first but the rewards and gameplay make it worthwhile. Also, if you get stuck, you can always consult the Handbook on the Web.
Vega Strike features excellent 3D graphics (developed, it should be remembered for free) and it has great potential for expansion thanks to the open support for new missions, new ships and endless mods.