Mitronix
Mitronix sells a multitude of lampholders including halogen, stage and theater lighting lampholders, fluorescent, mogul, medium screw, candelabra, and single contact bayonet lampholders.
Etlin
Etlin makes several types of lampholders including compact fluorescent, halogen (minican base or multibase), high output fluorescent (including a double recessed contact model), incandescent (phenolic, snap-in, porcelain, mogul), linear fluorescent and turret lampholders.
Leviton
Leviton makes fluorescent (compact and linear), HID/Mogul, incandescent (candle socket, keyless, multi-cluster, outlet box mount, and socket adapters), pulse rated, quartz halogen, and CFL lampholders.
Sylvania
Sylvania makes several types of lampholders with different strip lengths, wire lengths, wire types, and terminations. Types include tungsten halogen PAR, double ended, 2-pin polarized, miniature lamp, mini-can screw as well as many more.
Website: Sylvania
Buhl Electric
Buhl Electric makes halogen, fluorescent, compact fluorescent, incandescent, and HID lampholders. Their compact fluorescent base types include RX7S, E40, E39, G12, Fc2,and GY-22.Website: Buhl Electric
Hubbell
Hubbell makes several types of lampholders including floor boxes for wooden and concrete floors, incandescent lampholders made from plastic or porcelain, fluorescent lampholders, and socket extensions.Website: Hubbell