Stars Without Number is becoming one of my favourite Sci-Fi RPG's. It's game mechanics are simple to grasp, being easy and fluid in play, yet crunchy and satisfying enough to deal with nearly any Sci-Fi setting you want to stomp around in. The default setting is elegant and infinitely expandable. The actual rules are kept firmly in check, making combat deadly and intensely satisfying. Think twice before starting a fight. They end quickly.
If you have a hankering for some 2d6 Traveller style Sci-Fi, with some OSR/3.5 d20 style D&D combat mechanics, this is highly recommended.