Let's see....
These are the ones i play most for the time being:
Freespace: I agree with all that is written by other users; great story is one of the main features.
Half-Life: Same as written by other users.
Noctis: Why havn't this one been mentioned before?
Great atmosphere, over 70,000,000,000,000+ stars and planets to propsect and explore, free mobility; free to dream, impressive use of fractal-seed-engine, most addicting freeware-game in years.
Orbiter: Innoventive true-nevtonian-physics-engine, great add-on capability, great graphics (for a freeware game), great atmosphere, having the ability to explore all planets of Sol in real-time.
Countless others: Tough I like many other games as well, they are to many to list; and those listed here are those I play most for the time being.
Stargazer.