Star Frontiers
I loved Star Frontiers! Aside from some brief mentions in Dragon/Dungeon/Polyhedron, Alternity, and most recently, D20 Modern/Future, I've always hoped for an update that kept the spirit of that space opera game but updated and expanded it for D20. I played a Yazirian (flying monkey) who blew himself up while driving on Volturnus...
Sure, I've bought many D20 science fiction games, including licensed adaptations of Babylon 5, DragonStar, Farscape, Prime Directive (Star Trek), Serenity/Firefly, Stargate, Star Wars, and of course, Traveller, but the generic ones such as Bulldogs or Blood & Space come closest to enabling me to return to the excitement I felt when I first played Star Frontiers 25 years ago! It also inspired much of my GURPS campaigns, although True 20 might be a good rules base for a Star Frontiers game now.