Ralf Brown's Interrupt List - the definitive reference for MS-DOS interrupts.
Tenberry Software Inc. - Publisher of DOS4G/W. The DOS4GW FAQ is available here.
PMODE/W Home Page - home to latest version of PMODE/W, a free DOS extender that works with Watcom.
Devore Software & Consulting - home to Causeway a Dos Extender that works with Watcom. Very active development.