Some others:
Silent Legions, Godbound, Scarlet Heroes, Spears of Destiny, and Stars Without Number. Stars Without Number is getting a 2nd edition latter this years
Some other classics that weren't mentioned:
Top Secret (Note: The old classic version had 2 editions. Wrestling chart & value, and flamethrower all got pulled in 2nd version and some other stuff was added.)
Top Secret S.I.
Gangbusters
Star Frontiers/ Knight Hawks
Time Master and similar games that use the Pacesetter system like Star Ace, Rotworld, etc.
Villains & Vigilantes
Some additional info on some others already mentioned :
Starships and Spacemen has 2 different editions. The 2e version is compatible with LL, Mutant Future, and Apes. The older 1st edition isn't and is a different system. Goblinoid Games sell both. The 2e version is the more popular one..
Boot Hill had the old classic edition and a newer edition that came out years latter, which had changes to the system and is very hard to find.
Metamorphosis Alpha has had different editions and they have major changes in them in the way the game works.
Gamma World had different editions and the system is very different on the version being used. 1st & 2nd were compatible and the major changes in 2nd got introduced in 1st edition module. 3rd used a different system for resolution. and had skills, and talents. 4th was made to be compatible with AD&D 2e. Other latter editions aren't compatible at all with any older version.
Mutant Future is a retroclone of old 1st & 2nd Gamma World. Most people use that who can't get an older copy Gamma World and it has free version no-art version, a lot of support from various companies. This is the closest game to old Gamma World.
OSRIC did have a Fantasy Grounds version for and older version of Fantasy Grounds that doesn't work well in the new version of FG, due to changes, and and some modules. So it might be possible to use some of the old version on in making a new one.