Here are my three: pre-ban R6520 Gov't Carbine; post-ban LE6920HB Law Enforcement Carbine; post-ban MT6400C M4 Carbine.
GG&G single point sling end plate (all);
TangoDown pistol grip (all);
Troy Rear Sight (all);
Giles single point sling (the Gov't Carbine and LE Carbine);
TangoDown Vertical Foregrip (the Gov't Carbine and LE Carbine);Side Sling Mounts
(all, purchased one for Gov't Carbine from business listed at link);
KAC RAS ( the Gov't Carbine and LE Carbine);
MagPul rail protectors and KAC panels (the Gov't Carbine and LE Carbine);
Fire control and bolt upgrade from Specialized Armament
Badger Ordnance Tac Latch (all -- only way to go);
SureFire M952C (Gov't Carbine);
LaRue M68 CCO mount and Aimpoint M2 (Gov't Carbine);DIY conversion of fixed stock to true collapsible stock
Muzzle brake removed, barrel threaded and A2FH installed by ADCO Firearms
16.5" 6920 HB (LE Carbine); and
14.5" 6921 HB with GemTech BiLock (Gov't Carbine).
My thanks to ar15.com, Tina and Ken Elmore, and especially FRO for helping me get my "Arsenal of Democracy" up to speed.