Traveller
Traveller has been a little favourite of mine for decades. Both the game (in its various versions) and the milieu have plenty of flaws, mind you, but I liked the utilitarian simplicity of the former and the open-endness of the latter.

Unfortunately I never managed to play Traveller enough to satisfy my curiosity for it: I was a player in a short lived campaign using the venerable LBBs and refereed one myself using Gurps first, and my own T4 variant later. I put all I could "salvage" from my own campaign (including the rules) on a website, which I have now transplanted here (but the look and feel is the old one - beware!)

My ancient Traveller RPG pages - rehosted locally.