Current major systems would be d20, GURPS, WW's Storyteller system (World of Darkness, Exalted), and Eden's Unisystem (All Flesh, Buffy etc).
Edit: I would no longer count the BRP % System (Call of Cthulu, Elric, Runequest) but in its day it was certainly a major system. There have been others I'd have once been tempted to call major systems too, like the d20-based system GDW used for Twlight:2000, Traveller New Era, Dark Conspiracy etc. or WEG's d6 System (Star Wars).
The rest (Cyberpunk, Shadowrun, Riddle of Steel etc) I'd say were minor systems, being limited mostly to 1 or 2 games.