Press the ~ (tilde) to bring down the console, then type in these codes:

changelevel X	Changes level (X=3Dlevelname)
chase_active 1	Third-person view
give 2	Weapon number 2
give 3	Weapon number 3
give 4	Weapon number 4
give h ###	### (eg 999) health
God	God mode
impulse 9	All weapons and mana
impulse 14	Sheep
impulse 23	Torch
impulse 25	Tome of power
impulse 39	Fly mode
impulse 40	Level up
impulse 43	All weapons, mana and items
impulse 299	20 of each artefact
name X	Change your name (X)
noclip	No clipping mode
sv_gravity ##	Adjust gravity to number ##
Notarget	Toggle invisible
Restart		Restart the level