Complete the game to get an alternate title screen

Make sure the Developer Console is enabled in the options (Keyboard and Mouse -> Enable Developers Console), and assign a proper key to open the console. Open the developer console in-game and type in the following commands for the required effect:

Portals_ResizeAll - Changes portal sizes to what you specify (Replace and with half of the width and length you want on the portals)
sv_portal_placement_never_bump 1 - Makes sure your portals can be placed in any way you shoot them, even inside each other
sv_portal_placement_never_fail 1 - You can place portals wherever you want
sv_portal_placement_on_paint 1 - Makes portals on gels
portal2_portal_width XXX - Specify the width you want your portals to be, where XXX is a numerical value
portal2_square_portals - Make all portals entirely circle-shaped (Not long)
impulse 101 - Gives you all weapons (Including Half-Life 2 weapons)
Buddha - Makes you immortal (You never go below 1 health)
God - Makes you immortal (Your health is static)
noclip - Ability to fly, even through walls (Type again to toggle)
ent_create - Creates any entity you specify, where your crosshair is aiming
ent_create_paint_bomb_erase - Creates Water (To remove other gels)
ent_create_paint_bomb_jump - Creates Repulsion Gel paint
ent_create_paint_bomb_portal - Creates Conversion Gel paint
ent_create_paint_bomb_speed - Creates Propulsion Gel paint
ent_create_portal_companion_cube - Creates a Companion Cube
ent_create_portal_reflector_cube - Creates a Reflector Cube (For lasers)
sv_gravity XXX - Sets the gravity amount (600 = normal)
sv_cheats - Turns cheats on/off (0 = off / 1 = on)
notarget - Turrets dont target you
thirdperson - Changes view to third-person mode
firstperson - Changes view to first-person mode

There are more 'ent' commands and they are the same as those for Half-Life 2 and other Source games.
To see what you can spawn using ent_create, type listmodels in console.