From the main menu, go to Options -> KEYBOARD/MOUSE -> Allow Developer Console and set it to Enabled. Note: to make some of these codes work, you need to enter this code into the console: sv_cheats 1.

Effect	Code
Changes speed of game, 0.1 = 10% of original game speed, 2 = twice the original game speed	host_timescale # (default=1).
Changes view to first person	firstperson.
Changes view to third person	thirdperson.
Creates a Companion Cube	ent_create_portal_companion_cube.
Creates a reflector cube for lasers	ent_create_portal_reflector_cube.
Invincibility	god.
Lets you change the gravity in the game	sv_gravity # (600=normal).
Lets you go through walls and fly around	noclip.
Place portals anywhere	sv_portal_placement_never_fail # (1 or 0).
Player can take damage but won't die	buddha.
Shoots conversion gel out of your face	ent_create_paint_bomb_portal.
Shoots propulsion gel out of your face	ent_create_paint_bomb_speed.
Shoots repulsion gel out of your face	ent_create_paint_bomb_jump.
Shoots water out of your face	ent_create_paint_bomb_erase.
Shows FPS Rate	cl_showfps 1.
Turns Cheats on/off	sv_cheats (0=off/1=on).
Turrets won't shoot at you	notarget.