A couple of people have asked me what rules I currently use for my 15mm Sci-Fi gaming. Personally I'm a fan of the 'Beamstrike' rules, written by Neil Cooper. He has a dedicated web site, and in addition there is an active Yahoo web group with over 300 members.
'Beamstrike', a free, open source miniatures wargame developed for 15mm Science fiction figures. Inspired by 'Imperial Commander', Beamstrike rules are comprehensive, yet fast moving, there are ample examples of weapon systems, vehicles and equipment. You can use the background universe provided, yet the rules are generic enough to be used in your favourite science fiction setting.