One game that had entirely too much freedom was Daggerfall. It was great to have the freedom, but it got boring too fast because I kept falling off the plotline. In it's followup Morrowind it's easier to follow the plot. (Bought it the day after it came out, played it like 5 hours then got captured by Duke nukem Manhattan project and haven't played it for about 2 months now).
I tried installing Outcast but it never worked. The installer always crashed. Looked like a good game if you could get over the fact that it used voxel graphics.