Gas spore explosions should knock back any creature caught in them.

You can’t be knocked back if you’re carrying a loadstone.

If cast at Skilled or above, the spell of force bolt knocks back the first monster it hits (probably only 1 square like a monk’s staggering blow, or perhaps an Expert cast could provide additional knockback).

Wands of striking would not change since the Basic effects of the spell do not change.

Note that only two roles can cast force bolt at these levels: Knights (at Skilled) and Wizards (at Expert).

A #shove command, which allows you to shove a monster back one space, and will not anger most peacefuls, for the purpose of pushing peaceful monsters out of an altar room when they start crowding the altar you are trying to sacrifice things on. (Presumably, shoving a shopkeeper or priest or anything trying to stick to a specific location would not work the same.)

Artifact club The Pool Cue: it has the same damage as a regular club but causes several tiles of knockback to anything it hits.

Gnome punting: all kicks delivered to gnomes send them flying a few squares. (This also applies if a monster kicks the hero and the hero is a gnome.)

Ogresmasher can knock back (possibly entire groups of) small monsters it hits.

When a giant deals you a crushing blow and knocks you back, it can propel you into water you are flying or levitating over, wetting your inventory.

The player (and probably all creatures Large size or smaller) gets knocked back 1 tile when hit by a boulder (thrown or rolling).

Snow golem, which is a ’ monster found in icy locales. Does not generate in Gehennom. It uses a spitting attack to launch snowballs (flavored as throwing them), which are an object which behaves like spat venom in that they disappear one way or another at the end of their flight. These deal damage and possibly knockback. The encyclopedia entry for snow golems uses “Frosty the Snowman” as reference material.

If you are knocked back when levitating, you should fly about 3 times as far, since you’re only being slowed by air resistance and not by e.g. your feet on the ground. This maybe shouldn’t apply to when you throw items when levitating.

The player can shove things, dealing no damage but pushing the monster back. The monster can resist and fail to be shoved; it will resist a lot if the terrain you’re trying to shove it onto is bad for it (like water). You can only shove if you have hands, and you cannot do it to small or tiny monsters. Possibly an AT_SHOV attack should also be introduced for monsters to shove the player.

If you deliver a staggering blow or other knockback to a cockatrice, and there is a monster behind it, that monster might get stoned.

Spell of shove: level 4 or 5 escape spell. At Unskilled and Basic, it fires a beam that pushes the first monster it hits back one space (as if receiving a staggering blow, but doing no damage); at Skilled and Expert, it fires a shorter-range beam in all eight directions. What with shoving monsters into moats to drown them and things like that, this could be a pretty fun spell.

If you are wearing iron shoes, you cannot be forcibly knocked back to other spaces, unless you are levitating.

Add a knockback attack and a monster that can use it. Getting knocked back into a wall deals some extra damage, and getting knocked back into water or lava has predictable effects.

Carried loadstones prevent knockback, including Newton’s Third Law effects. Possibly it prevents levitation entirely.

Wind dragons, a cyan D whose breath is a wind beam: it doesn’t damage you, but blows you backward, and can’t be reflected. Their scales grant flight, which now confers resistance against knockback and Newton’s Third Law.

Bull rush command that lets a player shove a target monster back one or more squares. The relative size of the monster and player and the strength of the player are important factors. The character might move into the space of the displaced target.