Other Suns (out of print)
Was a fun system. Had many alien races as standard character templates. Used FL drives and Grav drives that worked differently than Traveller. FL was and could be much faster than Traveller times
Really like the Life Size points to determine how much lifesupport could substain.
Used as an empire on the other side of the galaxy. They got there via a major miss jump. Of course they (players characters) never made it back to Imperial space.
Justifiers (out of print and new updated version)
Fun, short, easy rules. Many many aliens and hybrids aliens. Humans also. Can't remember the 2 book miniature system that would work with it. Used many of the concepts for Corp Research in Traveller.
Mechniods (original prefered before Rifts Conversion)
Like the drawings, Really like the bent on why the Mechnoids existed. Had fun with the large ship (book 3 of the original) with some players.
Living Steel
Liked the world/concept used it as world for a series of adventures.
Battle Tech (Baron mechs from a magazine article)
Used it to represent several worlds in a pocket empire that survived the Long Night with only Jump 2/Jump 1 Tech and lost information on how they work but had the blue prints to build more of the same. Max weight was 15 tons and very slow. Most were like Traveller BattleSuits but 3 to 5 times bigger for the same characteristics.
SpaceMaster
Used the different locations and ships designs for a different look for different parts of the universe or special NPC's. Sometimes used descriptions of equipment that was similar to Traveller when players accessed a planet data base for information versus the official data base/library. Several times the players thought they had found new tech when all it was, was a culture difference in how they described potatoes/patatoes.
StarFrontiers
For the adventures to add some different flavor of style. Love Larry Elmore drawings also.
There are many more. If this was not what you were asking for, sorry.
Dave Chase