In the movie The Two Towers, Gollum leads Frodo and Sam to the Black Gate of Mordor, where they watch an army of humans marching past the Black Gate, and then inside into Mordor. I can't recall seeing these troops fighting in any future scenes in either The Two Towers or Return of the King (I think I would have recognized them because of their impractical helmets!) and the only human troops I can recall fighting for Sauron is the group of mercenaries that arrive by corsairs and the men who are part of the Oliphant brigade. Why show a human army marching into Mordor if they weren't going to make an appearance in any battle scene? What happened to Mordor's human army?
While not as plentiful as Orcs, and not as prevelant in the movies, Men took the field in the Battle of the Pelennor Fields:
Southward beyond the road lay the main force of the Haradrim, and there their horsemen were gathered about the standard of their chieftain.
The Haradrim were Men from lands on the southern border of Gondor. The Haradrim and Corsairs of Umbar were also those vanquished by Aragorn's host of the Dead, of which mention is made in the books but the battle was "off screen".
Men were also present at the Battle of the Black Gate:
But the Men of Rhûn and of Harad, Easterling and Southron, saw the ruin of their war and the great majesty and glory of the Captains of the West. And those that were deepest and longest in evil servitude, hating the West, and yet were men proud and bold, in their turn now gathered themselves for a last stand of desperate battle. But the most part fled eastward as they could; and some cast their weapons down and sued for mercy.
Finally, there were more armies of Men amassed behind the Black Gate, ready to continue the assault on the West.
Yet armies he had. As far as their eyes could reach, along the skirts of the Morgai and away southward, there were camps. ... '...These are Men not Orcs, or my eyes are all wrong.' Neither he nor Frodo knew anything of the great slave-worked fields away south in this wide realm, beyond the fumes of the Mountain by the dark sad waters of Lake Núrnen; nor of the great roads that ran away east and south to tributary lands, from which the soldiers of the Tower brought long waggon-trains of goods and booty and fresh slaves.
So basically Men were present in large numbers in Sauron's armies. They just didn't make in on screen in the movies.
All quotes from the Return of the King.
The Return of the King movie also shows a clip of the soldiers of Rhûn moving into formation when Grond sets up at the gate and charging in after the trolls.