Here are some of the developement codes and other secret stuff for Id
softwares DOOM.  These work in version 1.0 of the shareware release.

DOOM -DEVPARM -WART 1 %1 -SKILL %2

Replace the %1 with the level # you want to start at, and the %2 with the skill
level.  For example, 'DOOM -DEVPARM -WART 1 4 -SKILL 4' starts you on the
secret military base level, at the ultra violence skill level.  Note that you
won't have any extra guns, ammo, or weapons, however.

While playing the game, you can type in the following codes for the listed
effect

IDKFA - Gives you all the weapons and a full load of ammo.  Also puts your
armor at 200

IDDQD - Makes you invulnerable to all damage.

IDBEHOLD - Followed by various letters, invokes IR goggles, invisibility,
rad suit, and other goodies.  Follow the menu at the top of the scree for the
last letter.

IDSPISPOPD - No clipping mode.  Lets you walk through walls.  Makes your
graphics look pretty weird, but it's a good way to discover secret places
that you can't figure out how to get to otherwise.  Note that you can't pick
up any items while this mode is active.

IDDT - Use this while the map is onscreen, and the entire level will appear.
Type it a second time, and the map will be overlaid with the locations of all
objects currently active on the level.