I'm surprised nobody has mentioned Space Opera by Fantasy Games Unlimited. This was a great game and really incorporated the aspects of space opera vs. hard science fiction. If original Traveller was on the hard science end of the spectrum, Space Opera was definitely on the space opera end.

I don't have the books anymore, but I'm not likely to ever run a science fiction campaign again. I thought it hit a real good tone because I didn't prefer the hard science stuff at the time.