Besides GURPS, Hero and Savage Worlds, you have the big gold book version of Basic Roleplaying, and more recently things like Heroes and Hardships, and the recent generic version of Cepheus Engine.
More universals:
CORPS 2e, EABA 1e, EABA 2e, Plainlable Game System, Simply Roleplaying, FUDGE, Fate Core, Fate Accelerated, Cortex Prime, Tri-stat, Genesys,
There's also the question of how you want to count things like Fantasy and Modern AGE, which are not intended for true universal use, but are intended for broad use (and have the degree of optional and bolt on rules you'd expect for it), our the WOIN system games.
The term I prefer is "Genre Engines"... Classic Traveller can be seen as one, and thus Cepheus as initially released.
Also in there: Mekton II, Mekton Zeta, BESM (Big Eyes Small Mouth, a subset Tristat), Robot Warriors (Hero), Danger International (Hero), Justice Inc (Hero), Fantasy Hero (Hero), Magic World (BRP), Twilight2000 2.x/Dark Conspiracy 1.x (GDW House System), Web Game System... and that's still just a sampler - the ones that came to mind.
BESM can be seen as an example of how to tweak Tri-Stat for a specific genre; there are other Tri-Stat games as well as the standalone generic tristat.
Then, there are the house systems that never got a standalone generic core... Palladium's "Megaversal" engine, ICE's Rolemaster/Spacemaster/MERP/HARP/Cyberspace engine, GDW's "GDW's House System"...
GDW's house system was used for 4 games: Twilight: 2000, Dark Conspiracy, Cadillacs and Dinosaurs, and Traveller: The New Era. T2K+DC provides a pretty good coverage moderns game. Actually, 2... the first run used a 1d10 mechanic; the second a 1d20 one. ANd changed autofire. And the nature of attribute/skill interactions.