Ristin's Campaign Construction Center
D20 Modern and D20 Future are good systems in my opinion.
One touch I particularly like is that characters get AC based on their class, which means it’s not only a question of their equipment.
This really would be my first stop for doing D20 (DnD) style adventure in space.
Having said that, depending on the tone of the game I would be tempted by Eclipse Phase (for space horror), also Mechwarrior 2 (just probably no ‘mechs ;) ) or Shadowrun for a more realistic action adventure (where people actually fear taking a bullet and it’s not just ticking off some hit points).