We almost always refer to this as the “Cheat Menu”, for whatever reason
There is a nice section here on TCRF about it, but here are the codes for both PS1 and PC, for convenience:
For PlayStation, at the title screen, hold L1 and press Triangle, Left, Left, Right, Square, Up, Up, Left, Circle. If entered correctly, ‘Cheat Menu Active’ will appear at the top of the screen. During gameplay, press R2 + L2 simultaneously to bring up the cheat menu.
For Windows, at the title screen, hold A and then press T, Left, Left, Right, Space, Up, Up, Left, C. During gameplay, press Page Up and Page Down simultaneously.
hdc0 figured out how to activate this on PC, along with a bunch of other Cheats, in this classic SRC post
Here is a clip where Thermospore managed to bug it out and have it open in the Main Menu
limbus noticed that in an old April 1999 build of the game, Cheat Mode had additional options that are not present in the release version of the game. For example, there is a full “Level Select” as well as a “Set Level Complete” option (discord post)
- and hdc0 points out that version of the Cheat Menu is present in this PC demo
Select Hub
You can select 1 through 5. You spawn into the hub as if you walked out of SMP (double check this; there are multiple hub spawns)
Inventory Edit
Here are the maximum values you are allowed to give yourself in the Inventory Edit screen:
(though, as seen in Key, it is possible for the values to go higher than that)