Suits of armour stand in a number of places around the castle. They are empty, mere metal and leather, but they do seem to have some amount of magical animation and even sentience to them.
They seem to move about:
The people in the portraits kept going to visit each other, and Harry was sure the coats of armour could walk. (PS8)
A suit of armour at the top of the staircase with the trick step laughs at Neville getting stuck:
Neville’s foot had sunk right through a step halfway up the staircase. There were many of these trick stairs at Hogwarts; it was second nature to most of the older students to jump this particular step, but Neville’s memory was notoriously poor. Harry and Ron seized him under the armpits and pulled him out, while a suit of armour at the top of the stairs creaked and clanked, laughing wheezily. “Shut it, you,” said Ron, banging down its visor as they passed (GF12).
The suits of armour creak on their own (CS5, DH29)
There is a suit of armour near the kitchens (PS12)
A suit of armour stands next to the front doors of the castle (CS12)
The suits of armour were bewitched into singing Christmas carols, but didn’t know all the words:
Everlasting icicles had been attached to the banisters of the marble staircase; the usual twelve Christmas trees in the Great Hall were bedecked with everything from luminous holly berries to real, hooting, golden owls, and the suits of armour had all been bewitched to sing carols whenever anyone passed them. It was quite something to hear “O Come, All Ye Faithful” sung by an empty helmet that only knew half the words. Several times, Filch the caretaker had to extract Peeves from inside the armour, where he had taken to hiding, filling in the gaps in the songs with lyrics of his own invention, all of which were very rude (GF22). During the Christmas Eve visit to Godric’s Hollow in 1997, Harry hears Christmas carols being sung in the church as they pass which reminds him of Hogwarts and of Peeves “bellowing rude versions of carols from inside suits of armour” (DH16).
Strange lights shine inside for Christmas (PA11), sometimes including everlasting candles (HBP15).
They still moved on their own, but did so silently after they’d been oiled for the visit by Durmstrang and Beauxbatons (GF15)
A suit of armour stands in the corridor near the office of the Defense Against the Dark Arts teacher. It turned its head to watch Harry as he hid behind it (OP29).
Peeves trapped Mrs. Norris inside suits of armour a couple of times (OP30).
In the Battle of Hogwarts, McGonagall uses the spell Piertotum Locomotor to call the suits of armour and statues of the castle to animate and defend Hogwarts (DH30).