Wish is essentially a debug mode. Press command and w to access the wish dialog.

There is an official list of wish commands at:

Wish allows you to spawn ANYTHING. You can wish for "xp:<a number>" for a specific amount of xp.

You can force the engine to search first for a mutation by wishing for "mutation:<name>"

List of caveats:

Typing a number results in being teleported to a strange area that has its own overworld map.

The way data disks work doesn't really make the wish engine capable of finding a specific one, though you can wish for "smartass" to learn every recipe the wish engine can find.

You can spawn Chrome Pyramids.

You can spawn a merchant, kill him, and claim his random loot.

Weapons spawned via wishing have a chance to be "Flaming," "Freezing," "Counterweighted," etc...

Typing the plural of a word results in a random number of the object being spawned.

You can wish for mutations.

You can wish for "Gas" which gives you a mutation known simply as "Gas."