Half-Life: Team Fortress Classic

Change movement speed:
Note: This must be done on the server machine. Type sv_cheats 1 at the console, then type sv_friction <number>.

Level select:
Note: This must be done on the server machine. Type sv_cheats 1 at the console, then type changelevel <level name>.

See all monsters:
Note: This must be done on the server machine. Type sv_cheats 1 at the console, then type mp_allowmonsters. Restart the level to see the monsters. Note: This can be done when playing a map from another mod.

Change gravity:
Note: This must be done on the server machine. Type sv_cheats 1 at the console, then type sv_gravity <-9999-9999>.
Information in this section was contributed by Taylor.

Weapon select:
Note: This must be done on the server machine. Type sv_cheats 1 at the console. Change the map with the changelevel <map name> console command. Then, type tf_weapon_<weapon name> at the console on any machine connected to the server to select that weapon.
Information in this section was contributed by [FOS]US_Marine.

Weapon names:
Use one of the following entries with the tf_weapon_ code.

Object	Weapon name
Auto cannon	ac
Rocket launcher	rpg
Medic kit	medikit
Knife	knife
Super shotgun	supershotgun
Flame thrower	flamethrower
Railgun	railgun
Remote pipe bomb	pl
Grenade launcher	gl
Rocket launcher	rl
Tranquilizer gun	tranq
Railgun	railgun
Wrench	spanner

Shoot with crowbar:
Use the following steps to use the crowbar to shoot bullets from the chaingun or flame from a pyro. Change your class to an HW or Pyro. Then, use up all your ammo from your chaingun or flamethrower. After wasting all the ammo, get out the crowbar and hold the Right Mouse Button (or whatever displays your special ability menu) and run over a pack of ammo. You should now be able to shoot with your crowbar.
Information in this section was contributed by TheManInTheBigWhiteT-Shirt.

Select crowbar, then select the weapon you want to shoot. Use all of the ammo and hold the button. While holding it, press Q twice. While still holding it, get a pack of ammo. Release the button and you will be able to shoot with your crowbar.
Information in this section was contributed by EvilChild.

Dead scout sliding on the ground:
Change your class to a scout. Go in the console, type +reload, then go underwater in any level. Go in the console and type kill. Wait for someone to get your pack, then you will be a zombie. To respawn again, go in the console and type -reload. Note: This trick has been disabled in the latest versions of the game.
Information in this section was contributed by TheManInTheBigWhiteT-Shirt and Ryan Mills.

Keep same number of rounds after switching weapons:
Start a multi-player game and get the heavy weapons character. Have your Gatlin Gun ready and get ready to switch your gun. Have the gun that you want, ready then hold the Left Mouse Button for three seconds. You will hear the gun rotate. The gun you selected will have the same ammo that the other gun. This also gives you an extra 192 shots with the shotguns.
Information in this section was contributed by carcrasher3000.

Glitch: Invisible spy:
While in Team Fortress Classic, become a spy. Press [Ctrl] to crouch. Press [Right Click] (special ability) and select "Feign". If done correctly, you will have feigned like normal, but instead of being on the surface, you are now under it and are not visible by any other players. Once you want to get out of this position, just stop "Feign". Note: This is a good trick to do in an enemy's base, as you are able to communicate with your team. This is also a good time to change disguises, while no one can see you.
Information in this section was contributed by Foxdie54.