HED wheels and rims.
Salsa used to make quick releases, seat collars, bar-ends, and stems in the U.S.
Besides the hubs already mentioned, Phil makes bottom brackets and used to make pedals.
This is going a little far afield but Paragon Machine Works makes dropouts and various braze-on bits. Art Stump made the AD Stump dropouts.
I believe Wilderness Trail Bikes made a unicrown fork crown, also titanium handlebars, stems, "grease-guard" hubs and Charlie Cunningham's toggle-cam and roller-cam brakes.
OK, not USA, but North America and home to many of our esteemed members; Race Face made their cranks and other components in Canada.
Brent