Change movement speed:
You must do this on the server machine for this code to work. Submit sv_cheats 1 at the console, now submit sv_friction <number>.

Unlock Level select:
You must do this on the server machine for this code to work. Submit sv_cheats 1 at the console, now submit changelevel <level name>.

See all monsters:
You must do this on the server machine for this code to work. Submit sv_cheats 1 at the console, now submit mp_allowmonsters. Start the level over again to see the monsters.

Change gravity:
You must do this on the server machine for this code to work. Submit sv_cheats 1 at the console, now submit sv_gravity <-9999-9999>.

Weapon select:
You have to do this on the server machine. Submit sv_cheats 1 at the console. Alter the map with the changelevel <map name> console command. Now submit tf_weapon_<weapon name> at the console on any machine connected to the server to pick that particular weapon.
 

Weapon names:
Use one of the entries below when using the tf_weapon_ code.

Snipe farther:
When playing a sniper submit fov 10 at the console to double your sniping vision.